Britain is marking the tenth anniversary of the deadly Islamist suicide attacks on London's public transport system.
Fifty-two people were killed on July 7, 2005, when four young British men set off bombs in three subway trains and a bus during the morning commute.
Prime Minister David Cameron bowed silently Tuesday before laying a wreath in Hyde Park, where a memorial has been set up to honor the victims.
"Ten years on from the 7/7 London attacks, the threat from terrorism continues to be as real as it is deadly," the prime minister said in a statement.
Last month's massacre of 30 Britons and eight other tourists at a Tunisian beach resort was a "brutal reminder" of that danger, said the statement.
"But we will never be cowed by terrorism," it added.
Britain is currently at a "severe" security alert level, the second-highest of five terrorist threat tiers, meaning authorities deem an attack "highly likely."
In a rare public statement Tuesday, British spy chief Andrew Parker warned of the growing risk of "those who have grown up here but decided for whatever reason to identify their own country as the enemy."
"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," he said.
The terror threat has been amplified by the thousands of Westerners who have joined the Islamic State extremist group, which controls large parts of Iraq and Syria. There are concerns they could soon return to carry out attacks in their homeland.