They were joined by millions in offices, shops, factories and homes across the country to observe a two-minute silence.
It was the focal point of today's commemorative events to honour the victims of the suicide attacks, which killed 52 innocent people and left more than 700 injured.
Prime Minister Tony Blair observed the silence with members of the emergency services who led the response to the bombings.
The Queen, the Duke of Edinburgh, the Prince of Wales and the Duchess of Cornwall observed the silence at the Order of the Thistle service at St Giles' Cathedral in Edinburgh.
Home Secretary John Reid joined Metropolitan Police Commissioner Sir Ian Blair and his most senior officers outside New Scotland Yard.
As the nation came to a standstill, the start of the silence was marked in London by the chimes of Big Ben.
At the four blast sites, thousands of commuters, survivors and relatives of the victims stood solemnly, many with their heads bowed.
The silence was also observed at the Wimbledon tennis tournament.
The bell of St Paul's Cathedral marked the end of the two minutes.
Mr Blair said: "Today is a day of remembrance across London and the United Kingdom.
"It is a chance for the whole nation to come together to offer comfort and support to those who lost loved ones or were injured on that terrible day.
"This is a time when our country unites across all races, religions and divides and stands in solidarity with all those who have suffered so much in sympathy with them and in defence of the values we share."
A series of official memorial events began this morning at 8.50am - exactly one year to the minute since three of the four suicide bombers exploded their rucksack devices simultaneously on Tube trains across the capital.
At King's Cross, from where the four bombers fanned out that morning, Culture Secretary Tessa Jowell, London mayor Ken Livingstone, Transport Commissioner Peter Hendy and London Underground managing director Tim O'Toole laid memorial wreaths.
They then stood in silence, heads bowed.
At the same time, a bell tolled at St Paul's Cathedral and candles representing the first three blast sites - Aldgate, Edgware Road and Russell Square - were lit.
Nader Mozakka, 50, from North Finchley, north west London, who lost his wife Behnaz, 47, at King's Cross, was one of those who lay a floral tribute.
"I said goodbye to my wife at 7am a year ago and never saw her again," he said.
"It was tortuous for me to sit in a train today and it was tortuous for me to come here to King's Cross."
Just under one hour later, the commemorative events moved on to Tavistock Square, where 13 innocent people lost their lives in the bus explosion.
Mr Livingstone and Mr Hendy were joined by George Psaradakis, 50, the Greek-born driver of the number 30 bus that was ripped apart in the blast.
At exactly 9.47am, Mr Psaradakis laid a memorial wreath in tribute to the victims, reading: "You will never be forgotten. May you rest in peace."
At Edgware Road, where six innocent people died, the station manager organised a floral tribute of irises and carnations for the bereaved families.
Susannah Pall, 25, a City broker who was on the Circle line train which had a number of carriages destroyed in the blast, said she returned to the scene as part of the healing process.
Ms Pall, who has not travelled on the Underground since the bombings, said: "I've been building up to this day. I was going to stay away but decided to come.
"You've got to do these things. Until I do it, I can't move on - it's all part of the catharsis process."
At Aldgate, poems and messages of support were added to a burgeoning floral tribute outside neighbouring St Botolph church.
A book of condolence was placed on a lectern by the gates for passers-by and those affected by the tragedy to sign.
Carrie Taylor, a 24-year-old finance officer at the Royal Society of Arts, was killed in the blast. She came from Billericay in Essex.
A message from her relatives read: "Carrie, we will never forget you, rest in peace. Anne, Danny, May, Dave and David."
At Russell Square the station fell silent as transport staff, police officers and passengers remembered the 26 people who lost their lives on the Piccadilly line train last year.
Relatives and survivors who gathered outside the station tearfully hugged one another as they laid floral tributes to their loved ones.
Security was tightened across London today, amid fears that terrorists could try to use the anniversary to mount another attack.
But as the commemorative programme began, Britain's most senior police officer delivered a sobering warning.
Metropolitan Police Commissioner Sir Ian Blair said the terrorist threat had grown since last July.
He said: "Since July the threat has palpably increased and I fear that we have to accept that we live in an age for some years when the threat of an attack getting through is very real."
Ken Jones, president of the Association of Chief Police Officers (Acpo), also warned that the terrorist threat could last for "decades to come".
Memorial plaques were being unveiled this morning at the five main locations in London: Tavistock Square, Aldgate station, where Tanweer detonated his bomb, Edgware Road, where Khan detonated his, and at King's Cross and Russell Square stations - between which Lindsay exploded his device.
Later this afternoon, the bereaved families will attend a private ceremony at the Museum of London to place a book of tributes before a service at St Ethelburga's church in the City of London.
Hundreds of wellwishers were also laying flowers at a giant floral tribute to the victims in Regent's Park today.
The tribute, which is 40ft across, is in the shape of a flower with seven petals and was gradually being filled with purple carnations.
More than 1,000 people are expected at tonight's Regent's Park ceremony - the highest-profile event of the day.
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