The trial of US comedian and actor Bill Cosby for sexual assault has begun in Pennsylvania.

Former university employee Andrea Constand alleges he drugged and molested her in 2004.

Mr Cosby, 79, says he is innocent and the encounter was consensual. The trial is expected to last about two weeks.

It is being seen as the biggest US celebrity court case since the murder trial of former American football player OJ Simpson in 1995.

Several of Mr Cosby’s co-stars from the 1984-1992 sitcom The Cosby Show are expected to attend the trial in Norristown, outside Philadelphia.