A service is being held at St Paul’s Cathedral in memory of the 52 people killed by suicide bombers in London on 7 July 2005.
Survivors of the attacks and relatives of the victims are among those at the event. Many people across the UK also observed a minute’s silence at 11:30am.
Earlier, PM David Cameron and London Mayor Boris Johnson were among those who laid wreaths in Hyde Park – where another service will be held later.
Cameron 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 bombings of three Tube trains and a bus – carried out by four bombers linked to al-Qaeda and carrying rucksacks of explosives – was the worst single terrorist atrocity on British soil.
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At just after 08:50 on 7 July 2005, three explosions took place on the Underground – 26 people died at Russell Square, six at Edgware Road and seven at Aldgate.
Almost an hour later, a fourth device was set off on a double-decker bus in Tavistock Square, killing 13 people. More than 700 people were also injured in the attacks.
The names of the 52 victims will be read out during the national service of commemoration at St Paul’s.
The bombers began their journey to London from Luton. They were carried out by Mohammad Sidique Khan, 30, Shehzad Tanweer, 22, Hasib Hussain, 18, and Germaine Lindsay, 19. The group had links to al-Qaeda in Pakistan and Afghanistan.
