The 22 year old, who starred as bully Vincent Crabbe in the wizard franchise, is accused of handling the bomb during clashes in the Chalk Farm area of the city.
Prosecutor Rebekah Hummerstone also told Wood Green Crown Court the actor had swigged from a bottle of champagne which had been looted from a supermarket.
She said, "He accepted a bottle of sparkling wine from a friend who is also seen on the footage carrying a number of bottles... He agreed he had been handed a bottle of champagne to drink out of... He also agreed he had been carrying a petrol bomb and said he was asked by other people to carry it. He had some trouble with them in the past and felt obliged to comply."
According to police interview, the actor insisted he was "fascinated" by the riots which swept across the U.K. capital last August (11), but denied taking part in the violence.
Waylett denies charges of having an article with intent to destroy or damage property and violent disorder.
The trial continues.