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.
A co-founder of Help for Heroes has appealed for the public to support today's planned Hero Ride.
Bryn Parry said:
We are asking anyone who can, to join us in London (today), line the route of Hero Ride, donate and demonstrate your dignified respect and long-term commitment to those who serve and risk all on our behalf.
To our great British public, we thank you sincerely for your wonderful, continued support.
To ride alongside a young man missing three limbs as he powers his handbike up yet another hill, in the pouring rain, determined to complete a 90-mile day is inspiration enough to understand why our support is so needed, so important and must endure."
Hero Ride, a charity race organised by Help for Heroes, will be arriving in London later today, following a week full of money-raising efforts across the country.
Over 1,000 cyclists will meet in Blackheath, having completed their own individual charity bike rides, to cycle the final ten miles to the finish line at Horse Guards Parade.
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Listeners of LBC were invited to bid for the Mayor's personal, signed wristband along with the pants.
The bidding for the wristband and the underpants reached £205.
The Mayor described the charity as a "great cause" and vowed to sign the underpants for the lucky winner.
Help For Heroes has said it will not accept money from anyone donating to the charity "to promote their own political agenda".
A statement said this was because Help for Heroes "is a non-political organisation".
This evening, working with the assistance of JustGiving, we closed down a fundraising account which we believe the English Defence League intended to use to donate money to H4H.
We acted swiftly to achieve this, and would like to make it clear that we will not accept donations from organisations who attempt to use Help for Heroes to promote their own political agenda.
Like everyone, we were shocked by the murder in Woolwich. Like everyone, we just want to help our wounded servicemen, women, veterans and their families.
We hope that everyone who cares passionately about this important issue shows that support in a unified and dignified way.
Forces charity Help For Heroes has announced they will not accept any donations raised by the leader of the English Defence League (EDL) Tommy Robinson.
The EDL leader started fundraising following last week's murder of soldier Lee Rigby in Woolwich, south-east London, but a spokesman for Help For Heroes said the Just Giving page set up by Mr Robinson would be closed down and all donations would be refunded.
The charity will check for any further donations from EDL members and said they would not accept any donations from the group.
A Help For Heroes spokesman said: "He's the only one that's come to our attention but tonight we'll be doing a cross-count to make sure that anyone else that's saying they're EDL will not be allowed to fundraise for us.
"It's the same for any political party, we don't allow political fundraising. As a charity, we're non-political."
Forces charity Help For Heroes will not accept donations raised by Tommy Robinson, leader of the far-right group the English Defence League (EDL), Channel 4 News presenter Krishnan Guru-Murthy said.
It is thought that Drummer Lee Rigby was wearing a Help For Heroes t-shirt or sweatshirt when he was murdered in Woolwich last Wednesday.
The charity Help For Heroes has been swamped with donations, leading to its website crashing after Drummer Lee Rigby was murdered while wearing one of its tops.
Help For Heroes offered its thanks for the "extraordinary demonstration of support" which had taken it by surprise.
Posting a statement on its holding page, it said:
Since the sad news emerged that a serving soldier had been murdered in Woolwich, Help for Heroes has been overwhelmed with people spontaneously showing their support for the Armed Forces.
Our website is struggling to cope with this overwhelming reaction from the British public, some of whom are choosing to buy T shirts and hoodies.
This sudden surge of interest in the work we're doing to help the wounded and their families has taken us completely by surprise.
We just want to help, and all funds we receive will be used to provide direct, practical support to those affected by their service to our country.
We ask all our volunteers, fundraisers and donors to remember Lee Rigby's family, colleagues and friends.
Thousands of people have visited our website wishing to donate or to buy H4H T-shirts in an extraordinary demonstration of support and defiance of terrorism.
We are working hard to respond to this level of activity.
Prince William said Tedworth House made him and his brother, Prince Harry, "very, very proud" as he declared the recovery centre officially open.
He said: "It is an enormous pleasure to be here at Tedworth House. This place, and what Help for Heroes and its partners have done here, makes Harry and me very, very proud."