Britons marked the 10th anniversary of suicide bomb attacks on London's transit system Tuesday, as Prime Minister David Cameron said the recent slaying of 30 British tourists in Tunisia was a reminder that terror threats remain real and deadly.

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. Last month's gun rampage in the Tunisian resort of Sousse was the deadliest attack on Britons since then.

The head of Britain's MI5 domestic intelligence agency said the country faced a "serious societal and security challenge" from "a tiny fraction of the population" willing to use violence against their fellow citizens.

MI5 chief Andrew Parker said the 2005 bombings "and other 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."

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

He said Britain would continue to fight extremism, protect the public and promote "the shared values of tolerance, love and respect that make Britain so great."

Cameron will lay a wreath at a monument to 7/7 victims Tuesday before attending a memorial service at St. Paul's Cathedral in London.