Thousands of protesters from across the UK and around the world gathered in the rain in London today for a huge march for "jobs, justice and climate" in the first of a week of demonstrations ahead of the G20 summit.
Campaigners waving flags and placards braved wet weather to take part in the Put People First march, which comes amid anger at the £19 million cost of staging the conference at a time of economic downturn.
Today's march featured an alliance of 150 groups, including unions, charities, environmental campaigners and faith organisations.
A massive police operation was launched as officers warned of an "unprecedented" threat posed by the protests, but the organisers stressed today's events would not turn violent.
Climate Change Secretary Ed Miliband met campaigners before the march got under way this morning and said the "vast majority" were planning peaceful protest.
TUC General Secretary Brendan Barber, who was due to lead speeches at the rally, told the BBC Radio 4 Today programme: "If there are other groups who want to cause trouble, I don't want to see them anywhere near our event.
"I read about other events being planned by other groups over the next couple of days. If people want to make their protest in other ways, not the peaceful ways that we are absolutely committed to, then they should go in other directions."
The Bishop of London, Richard Chartres, said the atmosphere at the event was "positive" this morning, adding that it would be a "tragedy" if events turned violent.
He told Sky News: "It's not ideal weather for a demo this morning but nevertheless the atmosphere here is very positive indeed."
Asked if he was worried things could turn violent, he replied: "I very much hope not because that will undermine the message and the message is a huge number of groups united at a time of great difficulty in our own country, a time of redundancies, broken dreams.
"We've got to have some kind of constructive solution coming out of this meeting and it will be an entire distraction and a tragedy if it were marred by people piggybacking on the concerns of huge numbers of ordinary people."
Mr Miliband said that it was important that the demonstrators made their point peacefully.
"There is a very important line between peaceful and non-peaceful protest," he told the BBC.
"I think the kind of campaigning that people are engaged in today - certainly what the organisers intend, what the vast majority of the people who are going on this march will do - is the kind of peaceful protest that is very much part of our society."
People were descending on the capital from all over the country by train, car and coach. But travel within London was being seriously disrupted by engineering work on the London Underground which has closed the Victoria Line and partially closed four other lines.
An unprecedented alliance of 150 groups, including unions, charities, environmental campaigners, development and faith organisations were marching to Hyde Park for a rally with speakers and entertainment including Luke Pritchard of the Kooks, comedian Mark Thomas and Mauritanian musician Daby Toure.
Mr Barber will say at the rally there is "nothing inevitable about this recession" and that it is "right to be angry about the people losing their jobs and homes".
"Today is the birth of a powerful progressive voice," he will say.
"This is not the last time that we will work together, but just the first.
"And next week when G20 leaders come to London there will be voices for change. Voices that want to see global regulation of finance and a crack down on tax havens. Voices for a green new deal. Voices for the kind of economic boost that can fight unemployment and poverty."
At 8.30pm hundreds of millions of people around the world are expected to turn their lights off for one hour.
WWF's Earth Hour is aimed at showing global leaders the strong support for action to tackle climate change.
Landmarks including the London Eye, The Gherkin, and the BT Tower will take part.
But activists have warned companies in the financial sector they will shut down electricity supplies themselves unless the lights go out.
Further direct action is feared next week with police leave in the capital cancelled on Wednesday and Thursday, when campaigners are planning disruptions around the City's Square Mile.
A professor said to be masterminding "mayhem" has already been suspended by his university after he warned bankers could be "hanging from lampposts".
The University of East London confirmed a "full investigation" was launched following warnings of violence from Chris Knight, 66, from Lewisham, south London.
Glen Tarman, who has helped organise the march, said he had no evidence that anyone intended to break the law or commit acts of violence.
Murray Benham, head of campaigns at the World Development Movement, said: "The cost of the summit is clearly incredibly high. The only possible justification for this expenditure will be if the G20 leaders make the bold moves necessary to make the global economy work for people in the developing world, who are being hit the hardest by the credit crunch and climate change."
Among the marchers was the General Secretary of the Unite union, Derek Simpson.
He said: "This is about getting across the message that our members give to us about their concern over jobs and houses and the state of the economy.
"I think it's an important message but whether it will get through to the people meeting in London I don't know. Anyone who sees the numbers on this march should realise how important it is."
Police officers appeared to be keeping a relatively low profile at the march despite fears earlier this week that there may be clashes with some of the protesters.