Heath Ledger's family and friends have held a private memorial service for the Australian actor in Los Angeles, ahead of an expected funeral in his hometown of Perth.

Ledger's former fiancee Michelle Williams, the mother of his two-year-old daughter Matilda, and his ex-girlfriend Naomi Watts joined the star's parents Kim and Sally, and sister Kate, at the US ceremony.

"Even though it was a sad occasion, everyone was smiling, hugging each other and holding hands," a witness told American magazine Us Weekly. "It was a really positive group."

The service, at a funeral home in LA's Westwood Village, took place amid tight security after religious extremists threatened to picket it because of Ledger's performance as a gay cowboy in Brokeback Mountain.

Private funeral planned

Williams, Watts and other close friends are expected to fly to Western Australia early this week to mourn Ledger's passing at a private family funeral.

The Oscar-nominated actor, 28, died of a suspected accidental overdose in a New York apartment last week.

An autopsy proved inconclusive, and further tests are under way to confirm his cause of death, but his family insist the father-of-one would not have committed suicide.

On Saturday, a letter from Ledger's father Kim was read to stars at the G'Day USA Australia Day Ball, and a minute's silence was held in the Knight's Tale star's honour.

"Heath did not become an actor for the fame or fortune," the note, read by Australia's consul-general, Joh Olsen. "He loved his craft and he loved helping his friends."