Services have been held to remember the "ocean of pain" caused by the London bombings, in which 52 people died and more than 700 were hurt a decade ago.
A minute's silence was observed as survivors and relatives of the victims gathered at St Paul's Cathedral.
Petals fell from the dome as the Bishop of London said the Tube and bus attacks had united a city in "agonised outcry".
In Hyde Park, one survivor, Emma Craig, told crowds: "It may not have broken London, but it did break some of us."
The bombing of three Tube trains and a bus - carried out by four bombers linked to al-Qaeda carrying rucksacks of explosives - was the worst single terrorist atrocity on British soil.
At just after 08:50 on 7 July 2005, three explosions took place on the Underground - 26 people died at Russell Square, six at Edgware Road and seven at Aldgate.
Almost an hour later, a fourth device was set off on a double-decker bus in Tavistock Square, killing 13 people.
Ms Craig, who was 14 when she survived the blast at Aldgate, said: "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.
"It was and still is very much a part of my growing up, my childhood, my adolescence."
But Paul Dadge, who was photographed helping a survivor in the aftermath of the attack at Edgware Road station, said the terrorists would never win.
"They won't beat us because there is no point at which we will simply surrender to terrorism," said Mr Dadge, who was also speaking beside the 7 July memorial, which consists of 52 stainless steel pillars designed to symbolise the random nature of the loss of life.
"That's not the spirit we saw on 7 July. That's not the spirit we've ever seen. That's not the spirit we will ever see."
At St Paul's the Bishop of London, the Right Reverend Richard Chartres, said the Tube and bus attacks had affected people from all over the world.
"Soon after 7 July, the families and the friends of the victims compiled a book of tributes. It is a taste of the ocean of pain surrounding the loss of each one of the victims," he said.
"The tribute book is also very revealing about the character of the London which the bombers attacked. The majority of the victims were young - they came from all over the UK, all over the world.
"London is an astonishing world in a city. But beyond the diversity, the book also conveys a unifying, agonised outcry."
Candles representing the site of each incident were carried by people who helped deal with the immediate aftermath of the attacks including George Psaradakis, who was driving the bombed bus at Tavistock Square, and Dr Peter Holden, who provided urgent treatment at the scene.
The Duke of York attended the service and later the Duke of Cambridge joined victims' families, survivors and ambulance and fire brigade employees in Hyde Park for the tribute of songs and personal readings.
'I remember these Londoners, all walking'
By Emma Ailes, BBC News
Lins Drabwell, 35, walked 8.4 miles (13.5km) from her home in Bromley to her office in London Bridge. She said doing the "walk together" to mark the anniversary was "an easy decision".
"I remember 7/7 so clearly. Our office was near Edgware Road. We could see the station from our office window, and the emergency services going in," she said.
"I remember feeling that the attack was personal. It was my city, and my people. I felt very protective. That evening, I remember this mass of Londoners, all walking. It was very subdued.
"The next morning, I remember walking in seeing businessmen on the back of motorbikes, people who had dug out rusty old bikes, people on roller skates. There was a defiance."
Prime Minister David Cameron said the day of the attacks was "one of those days where everybody remembers exactly where they were when they heard the news" but he said "Britain would not be cowed by terrorism".
He and London Mayor Boris Johnson were among those who laid wreaths in a silent memorial at Hyde Park at 08:50 BST - the time of the first bombings.
In a note on his wreath Mr Johnson wrote: "Ten years may have passed, but London's memory is undimmed. We honour again today the victims of 7/7. You will live forever in the hearts of the people of this city."
Silences were also held across the London transport network and flowers were laid at the sites of the four explosions.
Carmen Macovei was among those caught in the blast at Tavistock Square.
She said: "I can still remember every moment of that day, except where I was sitting on the bus. I remember standing there after it happened thinking, 'What happened to my bus?' It still feels like yesterday."
'Fresh in memory'
Commuters were urged to "walk together" by finishing their morning bus or Underground commute one stop early and travelling the last few minutes by foot.
Adrian Luscombe, one of those taking part, tweeted: "A commuter today as I was 10 years ago. It could have been me. As fresh in memory as if it was yesterday."
The bombings were carried out by Mohammad Sidique Khan, 30, Shehzad Tanweer, 22, Hasib Hussain, 18, and Germaine Lindsay, 19. The group had links to al-Qaeda in Pakistan and Afghanistan.
Three of the men came from Beeston in Leeds and a crowd gathered in the area at the Hamara Centre to observe the minute's silence and a non-denominational ceremony was held at Leeds Civic Hall.
Hanif Malik, chief executive of the centre, said: "Ten years on from that atrocity we all vividly recall our shock, our horror, our dismay at discovering that Leeds was linked to those tragic events.
"The actions of those individuals 10 years ago were not reflective of the community here in Leeds and not reflective of the wider Muslim community."
Head of MI5 Andrew Parker said: "The terrible events in London on 7 July 2005 are enduring reminders of the reality of what MI5 is striving every day to prevent."
The UK's senior counter-terrorism officer, Metropolitan Police Assistant Commissioner Mark Rowley, said the rise of Islamic State militants in Syria and Iraq meant the UK was now facing a "very different" threat.
"We've seen another step change in terrorism in the way it works and connects across the world," he said.
The UK's terror threat level was raised from "substantial" to "severe" in August 2014 in response to conflicts in Iraq and Syria.
A total of 52 people lost their lives when four suicide bombers attacked central London 10 years ago. Here are their stories.