A film that cast unknown east London schoolgirls in most of its main roles has been honoured with five prizes at the British Independent Film Awards. Rocks won best British independent film, and one of its stars, Kosar Ali, 17, was named both best supporting actress and most promising newcomer.

The film got glowing reviews when it came out last year and is now on Netflix.

Its other winners included D'Angelou Osei Kissiedu, who was just seven when he played little brother Emmanuel. He was named best supporting actor, with a seven-decade age gap between him and the winner of the best actor award - Sir Anthony Hopkins, 83, who was recognised for playing a man struggling with dementia in The Father.

To find the young stars of Rocks, director Sarah Gavron and casting director Lucy Pardee auditioned about 1,300 students, most of whom had no acting experience. The chosen cast then improvised much of the film's dialogue. After the virtual awards ceremony, Ali, who plays Rocks' best friend Sumaya, said that the film had made her understand myself as a person as well as an actor.

She said: "Before this whole experience I never thought I would be who I am today, winning awards and even pursuing this acting career."

Producer Ameenah Ayub Allen said Rocks was a celebration of female film-making. "We were making this film that really had a truly independent spirit. She said: “It had a truly independent methodology".

"When we were filming, we always thought the film was imbued with the spirit of a teenage girl and it is just phenomenal that it has come here and that the spirit of a teenage girl has won best film, with a completely different way of film-making, with all these incredible women." Bukky Bakray, 18, who plays the title character, was nominated for best actress. That award went to Wunmi Mosaku for refugee horror His House, which had four wins in total.

The Father won three, while psychological horror Saint Maud won two. Actor Riz Ahmed claimed the award for debut screenwriter for Mogul Mowgli, which also won best music, while Ahmed's The Long Goodbye won best short film.