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.
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"Our ability to pre-empt or respond to threats will be eroded if we don't keep up with our adversaries."
"You are one of the silence breakers and we all owe you a debt of gratitude."