Mayor Sadiq Khan has laid a wreath at the 7 July memorial in Hyde Park to mark the 14th anniversary of the bomb attacks in London.
Fifty two people died and 700 were injured when four bombs went off across the capital city in 2005.
The heads of the British Transport Police, Metropolitan Police and City of London Police joined him at service.
In a speech, Mr Khan paid tribute to those who died as well as those who helped the injured.
"We will never forget those innocent victims, and as we grieve for them we also pay tribute to the heroic efforts of the emergency services and first responders who selflessly ran towards danger to help others," he said.
"Londoners showed resilience and unity in the face of huge adversity in 2005, and sadly our city has faced difficult times since then.
"But, standing together, we uphold the values that make this the best city in the world, united in defiance against terrorism."
Home Secretary Sajid Javid also paid his respects.
"We will never forget the 52 lives that were lost & the hundreds injured in the 7/7 attacks. Fourteen years on, our strength & resolve in the fight against terror remains undimmed. #WeStandTogether," he tweeted.