The event began with a performance from Alicia Keys in Los Angeles, followed by a welcome and an appeal to help the Haitian people by George Clooney.
Coldplay then performed their song 'A Message', beamed from their London studio.
Other artists still to appear include Madonna in New York City, Beyoncé, Sting, Christina Aguilera and Shakira.
The two-hour telethon is airing on all major US networks, MTV UK and online.
Stars including Robert Pattinson Jack Nicholson, Jennifer Aniston, Penelope Cruz, Billy Crystal, John Cusack, Stevie Wonder, Brad Pitt and Bill Clinton are taking donations.
All musical performances will be available to buy on iTunes from today. Proceeds will be donated to the Haiti earthquake relief funds.