Thousands of people protested on Saturday against Israel's siege of Gaza on the sidelines of the Venice Film Festival, seeking to move the spotlight from movie drama to real-world trauma.

Organised by left-wing political groups in northeast Italy, the demonstration began in the early evening a few kilometres from the festival where top Hollywood talent from George Clooney and Julia Roberts to Emma Stone have walked the red carpet in recent days.

The protesters marched slowly to the entrance of the festival in the beachfront Lido district, waving Palestinian flags, as the Hollywood blockbuster "Frankenstein" was due to have its world premiere nearby.

"You are all an audience to genocide," read one sign.

Protesters said the film industry should use its public platform at Venice -- the world's oldest film festival whose movies often go on to Oscar glory -- to focus attention on Gaza.

"The entertainment industry has the advantage of being followed a lot, and so they should take a position on Gaza," Marco Ciotola, a 31-year-old computer scientist from Venice, said.

"I don't say that everyone needs to say 'genocide', but at least everyone needs to take a position, because this is not a political situation. This is a human situation."

"We all know what is happening and it's not possible that it carries on," said Claudia Poggi, a teacher holding a Palestinian flag as people shouted "Stop the Genocide!" and "Free Palestine".

The Gaza war was one of the main talking points in the lead-up to the festival due to an open letter denouncing the Israeli government and calling on the festival to speak out against the war more forcefully.

The letter, drafted by a group called Venice4Palestine, has garnered more than 2,000 signatures from film professionals, including "Frankenstein" director Guillermo del Toro, according to organisers.

A similar initiative was organised at the Cannes Film Festival in May. (AFP)