Britain observes a minute's silence to remember the July 7 attacks amid a welter of warnings about the enduring and changing threat from terrorism a decade on.
The country fell silent to mark the 10th anniversary of the atrocity in which 52 people were murdered and hundreds more injured when four suicide bombers attacked London's transport network.
David Cameron said the Tunisian beach massacre showed the danger remains 10 years on but vowed the nation would not be cowed by violent extremists.
The Prime Minister said: "Today the country comes together to remember the victims of one of the deadliest terrorist atrocities on mainland Britain.
"Ten years on from the 7/7 London attacks, the threat from terrorism continues to be as real as it is deadly – the murder of 30 innocent Britons whilst holidaying in Tunisia is a brutal reminder of that fact. But we will never be cowed by terrorism.
"We will keep on doing all that we can to keep the British public safe, protecting vulnerable young minds from others' extremist beliefs and promoting the shared values of tolerance, love and respect that make Britain so great."
The anniversary falls at a time of heightened alert after the rise of Islamic State (IS).
Metropolitan Police Assistant Commissioner Mark Rowley, the country's most senior counter-terrorism officer, said security services are tackling a "very different" threat 10 years later.
In an interview with the Press Association, he called on communities to "step forward" in the fight against radicalisation, saying their help is more crucial than ever to security services' efforts to prevent fresh attacks.
Andrew Parker, the head of MI5, said the "continuing fact" that some British citizens are prepared to target their own country presents a "serious societal and security challenge".
He described the "disgusting" murders 10 years ago as an "enduring reminder" of what his organisation "is striving every day to prevent".
Tony Blair, who was prime minister at the time of the attacks, said his first response was to try to "bring people together" and deal with the "huge trauma" suffered by the capital.
Mr Blair denied that the terrorist attacks could be portrayed as a response to his foreign policy, telling LBC: "This is a global problem... and the only way of dealing with it ultimately is for people to come together whatever their faith background and say we are united against this terrorism, and to say we are not going to allow anyone to excuse themselves by saying the slaughter of totally innocent people is somehow a response to any decision by any government."
Metropolitan Police Commissioner Sir Bernard Hogan-Howe said it is a "day to remember and reflect".
He added: "We will never, ever be complacent. Whilst I hope that we will never need to deliver such a response again, if we do we will be ready.
"My thoughts today are with those taken from us, those who were affected, remain affected and with my own men and women who, day-in day-out are here for London."
Mayor of London Boris Johnson said today's remembrance is intended to show the bereaved, survivors and the emergency services that they have the support of all those across the city.
"Our priority today is to allow the families and friends of the 52 innocent victims of 7/7 to pay tribute to the memory of their loved ones," he said. "And for the survivors and the many hundreds of members of our emergency services who were affected by these atrocities to know that they have the support of every Londoner.
"On the tenth anniversary of the attacks we honour the victims, we remember the sufferings of their families and we pay tribute to the actions of our emergency services on that appalling day."
Events are being held to mark the anniversary, including a national minute's silence at 11.30am.
The period of quiet will take place during a service at St Paul's Cathedral - attended by the Duke of York, Mr Cameron, Mr Johnson, Mr Blair and Ken Livingstone, who was Mayor of London at the time of the attacks - and will be observed across the capital's public transport network.
Announcements will be halted and bus drivers asked to bring vehicles to a stop if they can do so safely.
Tube services will run as normal but passengers will be asked to observe the silence and announcements will be halted for the duration, Transport for London said.
Survivors, relatives of the dead and members of the emergency services have been invited to the event.
Wreaths will be laid beforehand at the permanent Hyde Park memorial, which Mr Cameron will attend, and a second service, to be attended by the Duke of Cambridge, will take place there later, featuring music, a series of readings and the laying of flowers.
Jacqui Putnam, who survived the Edgware Road explosion, said: "It doesn't seem like 10 years ago. There is only one group of people who I want to be with and that is my survivor family."
Ms Putnam, from Cambridgeshire, said her overriding feeling will be sadness as she remembers the 52 people who were killed.
"I think of them as my silent friends," she said. "Those 52 people are with me every day because I could so easily have been with them. The fact that I'm not is random luck."
July 7 2005 had dawned with London still elated from learning the previous day that it had won the 2012 Olympics, but within hours, the country was consumed by horror and grief.
Suicide bombers Mohammed Sidique Khan, 30, Shehzad Tanweer, 22, Hasib Hussain, 18, and Jermaine Lindsay, 19, met at Luton station that morning.
They took a train to King's Cross in London, hugged and separated to carry out their deadly missions.
Within three minutes of 8.50am, Tanweer detonated his bomb at Aldgate, Khan set his device off at Edgware Road and Lindsay blew himself up between King's Cross and Russell Square.
Hussain detonated his device on a number 30 bus at Tavistock Square at 9.47am.
Twenty-six died in the bombing at Russell Square on the Piccadilly line, six in the bombing at Edgware Road on the Circle line, seven in the bombing at Aldgate on the Circle line, and 13 in the bombing on the bus at Tavistock Square.
A fortnight later, another four would-be suicide bombers launched failed attacks on the Tube and a bus, leading to police marksmen killing innocent Brazilian Jean Charles de Menezes.