Britons laid flowers at the sites of the 2005 London suicide bombings and held a nationwide minute of silence for the 52 victims on Tuesday on the tenth anniversary of the attacks.

London: Britons laid flowers at the sites of the 2005 London suicide bombings and held a nationwide minute of silence for the 52 victims on Tuesday on the tenth anniversary of the attacks.

Prime Minister David Cameron led the tributes by placing a wreath at a memorial in Hyde Park and petals were released from the dome of St Paul's Cathedral during a service there.

The Hyde Park ceremony began at exactly 8:50am (0750 GMT) -- the time the first of four homegrown jihadists detonated his device on London's transport system on July 7, 2005.

"It's still raw 10 years on," said Mark, a 40-year-old train driver fighting back tears on the plaza outside King's Cross train station, near two of the four blast scenes.

"You see things you don't want to see again. It was horrendous," he said, adding that he was on duty at the time and was involved in rescue operations.

Bouquets of flowers were laid in nearby Tavistock Square, on the spot where one of the bombers detonated his device on a red double-decker bus, killing 13 people.

"Our precious daughter Shyanu. When heaven took our angel back, they left two broken hearts," read one message left in memory of 30-year-old Shayanuja Parathasangary.

Terrorism threat is 'real'

At Russell Square Underground station where a second device was detonated on a train, a tent was erected near the station entrance where mourners could sit and pay a silent tribute.

Spectators at the Wimbledon tennis tournament joined in the minute of silence, as did tourists outside St Paul's where families of the victims and survivors had gathered.

"Ten years on from the 7/7 London attacks, the threat from terrorism continues to be as real as it is deadly," Cameron said.

"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."

The Britons were among 38 people killed when a gunman went on the rampage at a popular Tunisian beach resort on June 26, Britain's worst terror incident since the 2005 bombings.

In a social media tribute that quickly trended on Twitter, many commuters posted pictures of themselves walking to work as part of a #WalkTogether campaign by leaders of different faiths to honour the victims.

In the past decade, successive governments have strengthened security powers and improved the way the emergency services respond to attacks after criticism of severe delays.

But they are still struggling to address the problem of radicalisation exposed by the bombings, which were carried out not by foreign fighters but by four young men who grew up in Britain and were inspired by Al-Qaeda.

Hundreds of British young people are now flocking to join the Islamic State group in Syria and Iraq, raising fears that they might return to attack their homeland.

'It broke some of us'

Despite new security measures put in place in recent years, the head of the MI5 domestic intelligence agency, Andrew Parker, warned that the risk remained high.

"Appalling acts are attempted by individuals who have grown up here but decided for whatever twisted reasons to identify their own country as the enemy," he said in a rare public statement.

"The continuing fact that some people, born in the UK, with all the opportunities and freedoms that modern Britain offers, can nonetheless make those sorts of warped choices presents a serious societal and security challenge."

For many of those directly affected by the London bombings, the anniversary has brought back painful memories.

David Boyce was a 25-year-old supervisor at Russell Square station and one of the first to witness the carnage.

"There were body parts all over the place and dead bodies lying all over the train," he told AFP.

Prince William joined the ceremony in Hyde Park where several survivors spoke of their experiences.

Emma Craig, 24, was just a schoolgirl on one of the trains targeted as she made her way to a work experience programme.

"All of us lost our innocence on that day," she said.

"It might not have broken London but it broke some of us."

AFP