Britons paused in silence and walked in solidarity Tuesday to mark the 10th anniversary of suicide bomb attacks on London's transit system.

Four British men inspired by al-Qaida blew themselves up on three London subway trains and a bus during the morning rush hour on July 7, 2005, killing 52 commuters and injuring more than 700.

Prime Minister David Cameron and London Mayor Boris Johnson led a wreath-laying ceremony at a memorial in London's Hyde Park at 8:50 a.m., the moment the attacks began. Survivors, transport staff and emergency workers laid flowers at the sites of the four bombings, while some Londoners heeded a call to walk part of the way to work — under the slogan Walk Together — in memory of the day 10 years ago when the city's transit network stopped.

Cameron said the recent slaying of 30 British tourists in Tunisia was a reminder that terrorist threats remain real and deadly. Last month's gun rampage in the Tunisian resort of Sousse was the deadliest attack on Britons since 2005.

"It's a day when we recall the incredible resolve and resolution of Londoners and the United Kingdom, a day when we remember the threat that we still face," Cameron said. "But, above all it's a day when we think of the grace and the dignity of the victims' families for all they have been through and we honor the memory of those victims and all those that were lost 10 years ago today."

A memorial service was also being held Tuesday at St. Paul's Cathedral in London, and later Prince William will attend a ceremony at the Hyde Park memorial.