On 1 July 1972, a group of around 700 lesbian, gay, bisexual and transgender (LGBT) people gathered in London's Trafalgar Square. They carried placards, waved banners and chanted loudly about how good it was to be gay. To the bemusement of the general public, some of whom pelted them with cans and coins, same-sex couples kissed brazenly in the street. "Aren't you ashamed?" one man shouted. "No!" came the reply as the marchers turned collectively and blew him a kiss. Then they all headed to Hyde Park, where they picnicked, smoked dope and played camped-up versions of party games such as spin the bottle.
Among the throng of demonstrators on what turned out to be the first ever Gay Pride march was a long-haired, fresh-faced 20-year-old named Peter Tatchell. He came arm-in-arm with his then boyfriend, 17-year old Peter Smith, a budding jazz guitarist. Tatchell, who had stepped off a plane from his native Melbourne just a year previously, was already deeply immersed in the capital's close-knit gay scene.
"Back then homosexuality was viewed as an illness. We were seen as criminal, immoral and abnormal," says Tatchell. "You could be arrested for kissing in the street and queer bashing was absolutely rife. To take to the streets like that was scary and nerve-racking. The whole event was very aggressively policed and there was almost one officer to every protester. Anyone who stepped a few inches out of line was roughly pushed back. The police were openly homophobic, abusing us and insulting us with impunity."
Only a few years after the Sexual Offences Act of 1967 declared that being gay was no longer an imprisonable offence, these were still dangerous times for the LGBT community in the UK; an era when lesbians routinely lost children in custody battles and electric shock therapy was dished out to cure homosexuals of their affliction.
The idea for Gay Pride was imported from the States, where a group of gay activists proposed taking to the streets of New York in 1970 in a show of defiance to commemorate the first anniversary of the Stonewall riots. These days, politicians on both sides of the Atlantic are falling over themselves to express their support for gay marriage.
Surrounded by the detritus of decades of frontline activism – placards, pamphlets and piles upon piles of paperwork in his tiny Elephant and Castle flat – Tatchell reflects on the achievements. "In the past 40 years we've gone from being outcasts and criminals to becoming mainstream and respectable," he says. "It's been one of the fastest and most successful social transformations in British history."
He remembers a time when the gay scene in London comprised a few basement bars with hatches built in to the doors so the owner could monitor who was trying to enter. "It was partly because people were closeted and fearful of being discovered and partly due to fear of attack. The scene was like an underground secret society. Most people knew each other."
But it was also an era that was both liberating and exhilarating. "We smoked dope, we tripped on acid, there was no HIV and the Tube cost 10p," says Tatchell. He remembers hanging out at the Salisbury pub on St Martin's Lane and the Sombrero in Kensington, "a small downstairs nightclub which had an illuminated flashing dancefloor where we danced to David Bowie, Marc Bolan and Donovan".
And, as the decades went on, so Pride grew. By the 1980s, its numbers had swelled to a few thousand, and it had moved, courtesy of Ken Livingstone and the GLC, to Jubilee Gardens on the banks of the Thames. "I remember the drag singer Divine sailing past on the river, blasting out her greatest hits," says Tatchell. "Boy George and Jimmy Somerville were regulars and Tom Robinson with his queer anthem – 'Glad to be Gay' – was always a favourite."
In 1988, the passing of the Conservative government's controversial Section 28 Act (which banned the "promotion" of homosexuality in schools) provided an unexpected boost to the cause. "Contrary to Thatcher's intentions, it was the making of the LGBT community. After that, numbers [at Pride] rose exponentially," says Tatchell.
Pride marches started springing up in cities across the UK, and reached a peak in 1997, when nearly 300,000 people turned up to the party on London's Clapham Common. "It got so big that for a few years there was an attempt by corporates to take it over and exploit it commercially," says Tatchell. "That was really bad for Pride and it was reflected in a fall in the number of people going."
Since then, Pride has clawed back some of its original spirit: the organisation is back under community control and political speakers are an important part of the bill. Though whether they have done enough to protect the original ethos is still a matter of hot debate.
But, for Tatchell, Pride has been a constant. He has rearranged holidays to ensure he hasn't missed one since the day it began. "I'm determined never to miss one until the day I die," he says. These days the event can be a bit hectic for him. He'll take his placard – last year's featuring "Pope 'Betty' Benedict XVI, the queen of homophobia" – and take to the streets. "I tend to get mobbed by reporters and well-wishers, which is nice but a bit tiring," he says. "Usually I go off to one of the satellite street parties in Soho afterwards, where I have a drink and a dance."
And does he, a man who has, after all, lived through dozens of bricks through his windows, two arson attacks and a bullet through his letterbox on account of his beliefs, envisage a time when Pride might disappear, rendered unnecessary by virtue of its own success? "According to the most recent British Social Attitudes survey," he replies, "36 per cent of the public believe homosexuality is 'mostly or always wrong'. My guess is we are going to need Pride for a few more years to come."
1970s: Nettie Pollard, 62, social research interviewer
"I attended the first Gay Pride in 1972. Going on demos was very exciting back then. The GLF [Gay Liberation Front] was flamboyant and the police were hostile; they considered us to be subversive. You have to remember that until 1967 gay men could be sent to prison. And lesbians were discriminated against in every way. There were no sex discrimination acts: lesbians routinely lost their children in custody cases. I remember walking down the street arm-in-arm with all the other lesbians and gays and seeing the amazement and bemusement of the general public. It was a magical atmosphere and quite ground-breaking. As time went on, Pride got bigger and bigger and it became more about wanting a place at the table than about changing society.
"In recent years we've had politicians trying to align themselves with Pride. Two years ago at the anniversary celebrations of the GLF, Boris Johnson, surrounded by heavies, barged his way to the front of the march.
"This year a friend of mine wrote to Pride's organisers to ask if there was a GLF contingent she could join to celebrate. 'I don't have any group like that in the parade,' came the reply. I think it's outrageous but not surprising in view of the way Pride has been going. It's now more a vehicle for politicians and corporate sponsorship than for sexual liberation.
"I'm still in touch with a lot of people from the early days. Practically none of us has become respectable. The establishment thought we should be in prisons, mental hospitals or at least hide our sexuality. Now it's utterly respectable to be gay, which is a bit sad in one way. Gays have become part of the establishment."
1980s: Simon Casson, 45, producer for performance-artist collective Duckie
"I think my first Pride was when I was 18, in Jubilee Gardens. I went with the tutors from my youth theatre group, who were all benders, of course – this was the mid-1980s, after all. I wore a pink triangle to let everyone know I was queer. It felt real and vulnerable and exciting, and I felt genuinely connected to it.
"Back then, the people who used to volunteer at Pride got a tab of acid as payment for their services. It was really symptomatic of the innocence and the communal ethos that lay at its roots – and back then we were all really into community activism. But then, in the early 1990s, the Gay Pride head honcho left and it became a free-for-all for all these capitalist huxsters.
"I remember going in 1995 and there were all these people dressed in Lycra, looking like a right state, blowing whistles and being really annoying. My mate Charlie said to me, 'If there was a march in the opposite direction called Gay Shame, you know what? I'd be on that one.' It really struck a chord. So we ran Gay Shame every year on the same day as Pride until 2009. We would mimic and take the mickey out of the mainstream gay scene. It was just a gag, but really it was just the same as Pride, in that it was a celebration."
1990s: Lucy Scher, 47, director of The Script Factory, a film-script development organisation
"I grew up in Camden in the 1970s and it felt as though anything went. Babies of the mid-1960s were born into a world in which it was much easier to be gay. We didn't feel it was a cross to bear. We were lipsticked up and having fun.
"I'm not sure Pride really changed my life, I've always been very much on the party side of the spectrum. I remember one year I designed a T-shirt with an E in a circle and the word 'quality' written underneath it. I printed 2,000 and they sold out in a second.
"In 1990, I was living in Manchester and started a clubnight at the Haçienda called Flesh. It was the first properly gay-straight night; I felt very strongly that I didn't want to turn away straight women. But there was a moment in the mid-1990s, and I daresay Flesh was at fault for this, that Pride was swamped by straight women. There was a lot of anger from certain elements of the gay community that it was being taken over and had turned into more of a party than a political movement. But I have always maintained that making it trendy was just a different way to equality.
"I went sporadically to Pride through the 1990s but as I got older I realised I couldn't bear the crowds and the panic about where to go to the loo. It all seemed way too difficult. But I believe in it and I love that people love it."
Noughties: Alfie Hubbard, 22, photography student
"The first Gay Pride event I went to was Leicester Pride in 2004. I was 14 and hadn't come out, but my best friend at the time was gay and he wanted to go along and I wanted to go with him to make sure he didn't get into any trouble.
"We grew up in the most crackpot small town in the Midlands and he was the only openly gay person there. He was severely bullied for it. We used to catch the school bus together and there was not one day when he wasn't verbally or physically attacked. He never retaliated; he sat there and smiled.
"Seeing him parade around the park and completely become someone else in the safe haven of Pride was amazing, and by the end of the day, we both just felt euphoric. We've been going every year since then and, in 2009, we went to the one in London which was astonishing – like being on another planet.
"One thing I've always said is that Pride is still a protest. People have become a lot more tolerant, but we are still not equal in the eyes of the law. Me and my sister share a very rare blood group, but I couldn't save her life if she was dying because I'm not allowed to give blood because I'm gay. I've got a scar on my forehead from where my head smacked the floor of the bus because someone tripped me up and called me a faggot. I think often people forget the true meaning of Pride. We need to keep reminding ourselves of why we are there."
This year's World Pride Parade Day is on Saturday. For details of the London event, see pridelondon.org