Hollywood megastar George Clooney was to be the star attraction at a Swiss fundraising gala for US Democratic presidential hopeful Barack Obama, a report said.

Around 170 US citizens have paid $US1,000 ($A1,180) a head to hear Clooney, fresh from his appearance at the Venice Film Festival, sing Obama's praises at the Geneva event, organisers told Swiss news agency ATS.

Clooney was also due to attend a private dinner at the home of lawyer and chief event organiser Charles Adams along with 75 other guests.

Organisers hoped the events would raise nearly $US1 million ($A1.1 million) for the Obama campaign.

Clooney is an avowed liberal and Democrat supporter and has long been a champion of Obama, the senator from Illinois who is hoping to oust the Republicans after eight years in the White House.

"I've been an Obama guy for a year-and-a-half and I'm very excited by his campaign," Clooney said in February.