Jason Sudeikis, Susan Sarandon and thousands of other actors have joined screenwriters for Hollywood's biggest strike in more than six decades.

Actors will not appear in films or even promote movies during the stoppage.

Major films in production, including the Avatar and Gladiator sequels, may be affected by the shutdown.

The actors are joining writers who walked out in May, concerned about pay, working conditions and the industry's use of artificial intelligence (AI).

Brian Cox, the lead actor on HBO's Succession, told the BBC the strike could last "until the end of the year".

"The whole streaming thing has shifted the paradigm," the Scottish star told BBC Newscast.

"They are trying to freeze us out and beat us into the ground, because there's a lot of money to be made in streaming and the desire is not to share it with the writers or the performers."

Talks for a new contract with studios and streaming giants broke down on Thursday, with the Screen Actors Guild (SAG) accusing the Alliance of Motion Picture and Television Producers (AMPTP) of being "unwilling to offer a fair deal".

About 160,000 performers stopped work at midnight, joining the 11,500 members of the Writers Guild of America (WGA), who walked out on 2 May.

By noon on Friday, union members and their supporters had gathered outside the offices of major studios and streaming services in Los Angeles, New York and other cities.

The demonstrations have received support from some of the biggest celebrities in the movie and television business, including the stars of the upcoming Oppenheimer movie, who walked off the red carpet on Thursday night.

The two guilds want studios and streaming services to offer better pay, increased royalties, higher contributions to their pension and health plans, and safeguards on the use of AI in the industry.

Productions likely to be affected include sequels to the Avatar, Deadpool and Gladiator franchises, as well as upcoming seasons of shows such as Stranger Things, Family Guy and The Simpsons.

Red-carpet premieres, promotional interviews and events including the Emmys and Comic-Con, have already been halted, rescheduled or scaled back.

The strike action is driven in part by an uncomfortable transition to the era of digital streaming, as well as by broader technological changes.

"AI will affect everybody," Academy Award winner Susan Sarandon told the BBC from a picket line in New York.

"There's definitely always been the feeling that if it isn't solved now, how do we ever solve it in the future?" she said.

"If you don't have the foresight to put something in place for the future, then you're screwed. It's clear that nothing is going to change from the top down, it's going to be up to us at the bottom."

Both writers and actors have complained that they make far less money than they used to make and that contracts have been undercut by inflation.

For actors, pay for individual roles has declined, forcing them to seek several more roles to make the same amount of money as they did a few years ago.

Writing contracts have become shorter and more perilous, with payment often not included for writers' work on revisions or new material.

"We are being victimised by a very greedy entity," Fran Drescher, the current SAG president, said on Thursday. "I am shocked by the way the people that we have been in business with are treating us."

The failed negotiations between the unions and the AMPTP marks the first tandem strike in the industry since 1960. The last actors' strike, in 1980, lasted 10 weeks.

A third union, the Directors Guild of America (DGA), is not participating in the strike after successfully negotiating its own contract in June, but the group has said it "strongly supports" those who are picketing.

With the prospect that the strike could roll on for months, cinemas could face problems, and viewers may be left with nothing new to watch bar reality TV and live sport.

In a statement on Friday, the White House said President Joe Biden "believes all workers - including actors - deserve fair pay and benefits".

"The President supports workers' right to strike and hopes the parties can reach a mutually beneficial agreement," spokeswoman Robyn Patterson said.

Actors represented by SAG's sister union in the UK - Equity - must continue to work as normal, due to UK employment laws. That includes stars of HBO's House of the Dragon.

The union has however told US companies it will be keeping a "very close eye" on any attempts to move US productions to the UK.