David Beckham, Kylie Minogue, Sebastian Coe and David Walliams are among the stars trying to raise £50m for charity as part of Sport Relief on BBC TV.
Sportspeople and celebrities are taking part in sketches, performances and challenges during the six-hour show.
Only Fools and Horses returns after 10 years with a sketch starring Beckham.
Walliams is grilling footballers in a special edition of Mastermind, while Lord Coe is taking on comedian John Bishop in a series of sporting events.
Gary Lineker and Davina McCall are among the hosts on the show, which aims to raise money for poor and disadvantaged people in the UK and around the world.
The programme, which is being
broadcast from London's Queen Elizabeth Olympic Park, is trying to beat the record £50.4m raised during the last Sport Relief two years ago.
Highlights of this year's Sport Relief include:
- Del Boy and Rodney are returning with a one-off Only Fools and Horses sketch featuring David Beckham
- Lord Coe and John Bishop are competing in sports including rhythmic gymnastics and synchronised swimming
- Kylie Minogue is performing her 1988 duet Especially for You "with a surprise singing partner"
- David Walliams has ousted John Humphrys as Mastermind host and is quizzing Arsenal's Jack Wilshere, Tottenham's Michael Dawson and Fulham's Scott Parker
- Four Paralympians have swapped their sports kit for sequins for a Sport Relief Does Strictly special
- Wimbledon champion Andy Murray is taking part in a Mock the Week special
Stars began their fundraising activities before the broadcast.
On Thursday, BBC Radio 2 DJ Jo Whiley completed a 26-hour stint on a treadmill. The donations included £260,000 from Coldplay.
A tweet from the band read: "Massive respect to the incredible, indegafatible [sic] @jowhiley £10K per hour makes £260K sponsorship from us."
Speaking afterwards, Whiley said: "I am blown away by Coldplay's generosity and it has made this challenge all the more sweeter."
At the climax of the gruelling challenge, Whiley stumbled following a final 10-second countdown. "I can't believe I did 26 hours and then fell at the last minute," she said.
On Monday, Radio 1 host Nick Grimshaw completed a 12-hour bike ride after he, along with other celebrities, cycled more than 1,000 miles and raised more than £115,000.
Davina McCall raised more than £750,000 by running, cycling and swimming from Edinburgh to London in just seven days.
Elsewhere, former England captain Alan Shearer beat ex-footballer Robbie Savage in an exhausting race to sit on all 90,000 seats in Wembley Stadium.
And comedian Russell Brand will become a pundit on BBC One's Match of the Day show on Saturday night.