The four blasts killed 52 people and injured more than 700
Hundreds of people crowded into King's Cross station to remember the 52 people killed in the 7/7 suicide bombings in London three years ago.
Minister for London Tessa Jowell and London Mayor Boris Johnson laid flowers there at 0850 BST, the time the first three bombs exploded.
Survivors and friends also attended low-key services at the sites of the other three blasts.
Four bombers detonated bombs on three Tube trains and a bus in 2005.
The four bombers set off on their terror campaign from King's Cross, before detonating devices near Russell Square, Aldgate and Edgware Road tube stations and on a bus in Tavistock Square.
Bombers Mohammad Sidique Khan, 30, Shehzad Tanweer, 22, and Hasib Mir Hussain, 18, all lived in West Yorkshire while Germaine Lindsay, 19, was from Buckinghamshire.
On Monday morning the pavements around King's Cross were crammed with commuters as passers-by stopped to pay their respects.
Mr Johnson laid a memorial card which said: "We honour the memory of those who died on 7/7 2005, we salute the courage of those who were injured and our thoughts and prayers are with all victims and their families."
Daniel Obachike, 34, from Finsbury Park, north London, was there to remember his best friend, Christian Small, who died in the blast.
He said: "I was with Christian the morning before he died - and obviously today feels particularly poignant."
Survivor Jacqui Putnam, 57, was in the first carriage when the bomb went off in the second carriage of the train at King's Cross.
She said: "I return every year to lay flowers. It's very important for me to be here. I don't want to be anywhere else on the 7th. The other people here today have become friends since."
More than 700 people were injured in the blasts and concern has been raised that 73 compensation claims have yet to be paid out.
Mr Johnson has written to Ms Jowell asking the government to ensure the backlog is dealt with immediately.
The Criminal Injuries Compensation Authority (CICA) said £7.5m had been paid to victims and relatives.
A spokesman said: "While we aimed to resolve all these applications as soon as possible, in some cases we still need information from third parties, for example, the police, medical specialists and employers."
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