More than 1,300 cyclists are taking to central London's streets for today's Hero Ride, in aid of the armed forces charity Help for Heroes.
They include 300 fundraisers who arrived in the capital from Paris, having cycled from France over five days.
Those taking part are cycling more than 350 miles through the battlefields of the First and Second World Wars.
Other self-organised rides are taking place from various locations and meeting in Blackheath, southeast London, before culminating at Horse Guards Parade.
More top news
The friends of a Czech businessman who was beaten to death raise £10,000 in a day to send body of 'great man with a big heart' back home
The Olympians rounded off their success in Rio with a personal triumph as they confirmed they have tied the knot.
The foreign secretary called for tougher action against Russia in an effort to halt the "barbaric" attacks on rebel-held Aleppo.