A record-breaking team is hoping to visit every Tube station within 20 hours to raise £30,000 for families of the London bombing victims.
Led by current record holders Geoff Marshall and Neil Blake, the team left Amersham, Bucks, at 0528 BST on Thursday and will pass 274 stations.
Along the journey they will be handing out flyers explaining the work of the London Bombings Relief Fund.
Mr Marshall said it was to show "We are not afraid to ride on the Tube".
"It's not about breaking records, it's about getting people, young and old, out and about.
'Not afraid' T-shirts
"There should be a good spirit and we want to spread the message of solidarity and defiance," he said.
The group of 67 will travel together for two hours before splitting off into teams, all aiming to finish at Upminster in east London between midnight and 0100 BST on Friday.
They will be wearing "not afraid" T-shirts, inspired by the website set up to give Londoners a voice in the wake of the bombings.
Volunteers will be based at a "control centre" at Euston where participants can get refreshments, first aid, or check on the progress of the others.
Mr Marshall, 33, from Epsom, Surrey and Mr Blake, 32, from Flitwick, Bedfordshire, recorded a time of 18 hours, 35 minutes and 43 seconds in May 2004.