Wreaths have been laid in memory of the victims of the 7/7 terror attacks on the 15th anniversary of the bombings.
The Mayor of London and the head of the Metropolitan police laid wreaths at a memorial to the attacks, in Hyde Park.
Fifty two people died and 700 were injured when four bombs went off across the capital city on 7 July 2005.
The heads of the Transport for London, British Transport Police, and City of London Police also joined the service.
Sadiq Khan, Mayor of London, said: "Our capital will never forget the terrible events of that day.
"As we mark 15 years since the attack on our city, I want again to pay tribute to the heroic efforts of our emergency services and transport workers, who ran towards danger to save lives, on that awful day.
"The way that our city responded and stood united in the aftermath of the attack showed the world that our values of decency, tolerance and mutual respect will always overcome the hate of the terrorists.
"Today, we reaffirm our commitment to upholding these values."
Dame Cressida Dick, head of the Met, and Mr Khan laid wreaths at the 7/7 memorial in Hyde Park at 08.50 BST on Tuesday morning - the time three bombs were detonated simultaneously, at Aldgate, Edgware Road and Russell Square underground stations.
A second group, including London Fire Brigade Commissioner Andy Roe and London Ambulance Service chief executive Garrett Emmerson, then laid wreaths at 09:47 - when further device exploded on a bus in Tavistock Square.
A separate virtual service for families of the 52 people who died in the 7 July bombings and survivors is being held later.
Paralympian Martine Wright, who lost both her legs in the Aldgate explosion, tweeted: "15 yrs ago I jumped on a circle line tube and it changed my life forever.
15 yrs ago I jumped on a circle line tube and it changed my life forever. Remembering all those affected by #londonbombings all those that risked their lives helping others and all 52 souls that lost their lives. My thoughts are with them and their families today and everyday 💕— martine wright MBE (@martine_wright) July 7, 2020
Dame Cressida said: "Today, I think also of my officers and staff who ran towards those terrible scenes. Who put themselves in danger and tried to do their best to support those affected.
"On behalf of everybody in the Metropolitan Police Service, we will not forget."
Mike Brown, Commissioner for Transport for London, said: "We will never forget those innocent victims who lost their lives in the most tragic circumstances 15 years ago.
"The resilience of great world cities like ours continues to be tested but Londoners have shown time and time again that our strength lies in our diversity, resourcefulness and spirit of togetherness."