Visibly moved survivors recounted the events of the day precisely 10 years ago as crowds heard readings and flowers were laid at the central London monument to the tragedy.
Some 400 people were seated at the service, and Prince William stood and chatted to many of them when the service was over.
Survivor Emma Craig, who was aged 14 when she was caught up in the Aldgate blast, wept as she spoke to the crowd.
Tribute: The Duke of Cambridge lays flowers at the 7/7 memorial in Hyde Park (Picture: PA)
Emotional: 7/7 survivor Emma Craig wipes away a tear as she addresses the crowd (Picture: PA)
"All of us lost our innocence on that day, our naivety, the thought that 'something like that could never happen to me' or even to London," she said.
She recalled her mother ringing her to check she was okay as she had heard about a bomb going off, and she remembered saying to her mother: "Mum I was there. I was on the Tube."
Ms Craig, who was uninjured, said her mother does not swear and recalled her saying: "Sugar. I'm on my way."
Prince William spoke to memorial attendees after the service (Picture: PA)Tracy Russell, from the London Ambulance Service, recalled the events of July 7 2005, remembering her walk through the Tube tunnel at Russell Square and the "smoke catching in our throats, adrenaline pumping through our veins".
She said: "This is a day that I never wish to know again."
The Duke sits with London Bombing Relief Charitable Fund chair Gerald Oppenheim to his right and Labour MP Tessa Jowell to his left (Picture: PA)Prince William, dressed in a dark suit, carried yellow flowers up to the memorial at the end of the service.
A note, next to the flowers that looked like the bunch carried up by William, said: "In memory of those who lost their lives and those injured in the atrocities of 7th July 2005."
Additional reporting by PA