An unofficial ceremony is being held in Hyde Park to mark the fifth anniversary of the 7 July bombings in the capital.
Survivors and families of those who lost their lives in the terror attacks laid flowers by 52 steel pillars which represent those killed.
A one-minute silence was observed and a wreath was laid in the name of Prime Minister David Cameron, who did not attend the service.
Some survivors and relatives are upset no official ceremony has been staged.
Four suicide bombers detonated explosives on board three Tube trains and a bus during the morning rush hour in 2005.
In a hand-written message attached to the wreath Mr Cameron said: "In memory of the victims of terrorism in London on 7 July 2005. They will never be forgotten."
But Graham Foulkes, who lost his 22-year-old son David in the Edgware Road bombing, criticised Mr Cameron's absence.
He said: "I don't think any of us are saying we want this to become an annual major event, but I think on the fifth anniversary the least the prime minister could do is attend and lay a wreath.
"This was a national attack, and it's really disappointing."
Contrasting the anniversary with the way the US marks the 9/11 attacks, he said: "The mindset of New Yorkers and the authorities in New York is completely different to here. Here we are at a significant anniversary and even the mayor can't be bothered to attend."
Lawyer Thelma Stober, who lost part of her leg in the Aldgate bombing, said: "We asked the DCMS [Department for Culture, Media and Sport] what they were going to do for the fifth anniversary and they said they were not prepared to do anything because there will be plenty of anniversaries."
However, a spokeswoman for the DCMS said: "Many of the families have told us that after five years, they no longer look to government to lead the commemorations. They prefer to remember their loved ones in their own way."
A spokeswoman for London Mayor Boris Johnson said he had followed the advice of the DCMS.
A wreath has been laid at Hyde Park on his behalf and a minute's silence will be observed at City Hall later, she added.
In a message to staff at the Greater London Authority, Mr Johnson said: "I know that this organisation rose to the challenge on that day and worked hard to keep the capital moving and its communities united in grief, not in mutual hatred or suspicion.
"We will never forget what happened or those who suffered."
Acting Labour leader Harriet Harman said in the Commons: "We stand with the government in our determination to defeat those who would bring terror to our streets."
The Muslim Council of Britain will lead a delegation from the community to pay their respects at the Hyde Park memorial.
Elsewhere, events marking the attacks will be held at Chatham House in London - where Security Minister Dame Pauline Neville-Jones will speak - and at the Makkah mosque in Leeds.