The Prince of Wales joined survivors and the bereaved at a memorial ceremony in Hyde Park to mark the 20th anniversary of the 7/7 attacks on London.
Four co-ordinated attacks on three Tube trains and a double-decker bus in 2005 killed 52 people and left several hundred injured.
The attack by four suicide bombers was the worst single terrorist atrocity on British soil.
A series of commemorative events were held on Monday to remember the victims killed and injured.
In the afternoon William joined a number of relatives of those who died as well as survivors at the event at the July 7 memorial gardens.
The mother of Carrie Taylor, 24, who was killed in the blast on a Circle Line Tube train near Aldgate station, read the words on her daughter’s headstone to attendees: “Missing you is easy, we do it every day. Remembering you is heartache that will never go away.”
Thelma Stober, who survived the Aldgate blast, losing her hearing after shrapnel caused by the explosion were found lodged in her brain, said: “20 years ago my life and those of many others was changed forever in ways we never imagined when we left our homes.”
She became emotional as she told the audience: “You will always have a hole in your family. Nothing I say can bring them back. They will live in our hearts. They will never be forgotten. May their souls rest in peace.”
Music at the service included Something Inside and Sometimes I Feel Like A Motherless Child performed by the London Gospel Choir. William laid flowers in tribute.
The Hyde Park Memorial Service followed a morning ceremony at St Paul's Cathedral, where the Prime Minister and the Mayor of London were among those to pay their respects.
Sir Keir and Sir Sadiq were joined by members of the Royal Family, the Home Secretary Yvette Cooper, survivors, emergency responders and bereaved relatives at the commemorative ceremony.
The daughter of a woman killed on 7/7 broke down as she read out her name during the service.
Saba Edwards and Ms Stober read the names of those who died in the attacks. The organ played softly in the background while 52,000 white petals fell from the ceiling of the dome.
Ms Edwards' voice cracked as she started reading the names of those who died, and when she got to the name of her mother Behnaz Mozakka, who was killed on the Piccadilly line , she began to sob and had to briefly pause.
Ms Stober took over and her voice also strained as she read out the names.
The women, both dressed in orange, turned to one another as the final petals fell and the cathedral fell into a minute of silence.
The service was opened with a bidding prayer read by Dean of St Paul's Cathedral the Very Reverend Andrew Tremlett, who said: "We gather in this Cathedral Church to mark the solemn anniversary of an act of terror inflicted on our city on this day 20 years ago.
"Together with friends from many other regions and nations, we continue to commemorate a moment in time that left deep scars in the soul of our capital.
"We come to honour the memory of those who died, to stand alongside the survivors, to give thanks for those who responded with courage and compassion, and to renew our shared commitment to peace, justice, and reconciliation.”
Earlier the Prime Minister and Sir Sadiq laid wreaths at the July 7 memorial in Hyde Park at 8.50am - to coincide with the time that the first bomb went off.
They were joined by officials including Metropolitan Police Commissioner Sir Mark Rowley and Chief Constable of British Transport Police Lucy D’Orsi who also laid floral tributes.
In a statement issued to mark the anniversary, Sir Keir said: “Today the whole country will unite to remember the lives lost in the 7/7 attacks, and all those whose lives were changed forever.
“We honour the courage shown that day — the bravery of the emergency services, the strength of survivors and the unity of Londoners in the face of terror.
“Those who tried to divide us failed. We stood together then, and we stand together now — against hate and for the values that define us of freedom, democracy and the rule of law.”
Mayor of London Sir Sadiq said Londoners “will always choose hope over fear and unity over division”.
“We remember the 52 people whose lives were cruelly stolen on that dark day and our thoughts and prayers remain with all of their loved ones, as well as the more than 700 people who were injured by these cowardly acts of hatred.
“We also pay tribute to those who ran towards danger on that terrible day.
“From emergency service personnel, to transport workers and members of the public who showed incredible courage and compassion by putting their own safety at risk to help those in need, their bravery and selflessness will never be forgotten.
“Two decades on – as we unite for a day of commemoration – I have a clear message for those who seek to spread division and sow hatred – you will never win.
“London’s determination to stand together is stronger than ever. We will always choose hope over fear and unity over division as we continue building a safer London for everyone.”
In his message to mark the anniversary, the King said comfort can be taken from the “spirit of unity” in London and the country more broadly, which has allowed the nation to heal.
He said: “Today, as we mark 20 years since the tragic events of 7th July 2005, my heartfelt thoughts and special prayers remain with all those whose lives were forever changed on that terrible summer’s day.”
“We remember with profound sadness the 52 innocent people who were killed in senseless acts of evil – and the enduring grief of their loved ones.
“We recall, too, the hundreds more who carry physical and psychological scars, and pray that their suffering may ease as the years pass.”
“In doing so, we should also remember the countless stories of extraordinary courage and compassion that emerged from the darkness of that day.
“The selfless bravery of our emergency services, transport workers, and fellow citizens who rushed towards danger to help strangers reminds us of the very best of humanity in the face of the very worst.”
The King also stressed the importance of communities coming together in times of adversity.
He said: “While the horrors will never be forgotten, we may take comfort from the way such events rally communities together in solidarity, solace and determination.
“It is this spirit of unity that has helped London, and our nation, to heal.
“As we remember those we lost, let us, therefore, use this 20th anniversary to reaffirm our commitment to building a society where people of all faiths and backgrounds can live together with mutual respect and understanding, always standing firm against those who would seek to divide us.”
In her address at the Hyde Park service, Ms Stober spoke of how vital it was society learnt lessons from the tragedy in training, support systems and legislation.
She said: “Caring for victims of terrorism is also a powerful weapon in the fight against it.
“The societal and community values terrorists fight against grow stronger under attack.”
