Cinema workers have staged a protest at the start of the BFI London film festival in defiance of a threat by their employer, Picturehouse, to sack those taking part in strike action.

About 50 protesters took part in the demonstration in Leicester Square, in the capital, as part of a campaign entering its second year demanding a London living wage.

Staff from five cinemas – Crouch End, Hackney, East Dulwich and Central Picturehouses, and the Ritzy – were on strike Wednesday afternoon and are due to walk out again from Friday to Sunday and from 11-15 October.

In a letter sent to the union Bectu last week, seen by the Guardian, lawyers representing Picturehouse said: “Any employee taking part in those strikes is likely to be dismissed.”

A spokesperson for the company appeared to backtrack on the warning, refusing to confirm whether or not striking workers would be sacked.

Picturehouse said in a statement: “After legal advice, we believe the current strikes would be unlawful and have informed Bectu of our view. We believe our staff will not participate in any unlawful action against the company.”

Alongside higher wages, workers want sick pay and the reinstatement of several union representatives who were sacked this year after taking part in industrial action.

Some protesters have accused Picturehouse of setting up a “fake” union to try to manage the dispute. Naomi Taylor, 34, a Bectu organising official, said: “Picturehouse set up a fake union. It’s not legally independent, it’s basically a glorified staff association … to keep out trade unions.”

Nesa Kelmendi, 23, a protester working at Central Picturehouse, when asked whether she was concerned by the threat of being sacked, replied: “Originally we were. It’s happened in the past with our four union reps, so we’ve seen that it’s possible. But at the same time they [Picturehouse] have pretty much got nothing to stand on.”

She added: “At the moment our bosses don’t really want to negotiate with us, they don’t want anything to do with us. They will only negotiate with people who are part of Forum, which has been set up by the management – it’s funded by the management and it’s run by the management.”

The pay campaign has been backed by a number of high-profile figures including the shadow chancellor, John McDonnell, and other Labour MPs. Twenty-five actors and directors including Sir Ian McKellen and Ken Loach have urged the public to boycott Picturehouse cinemas and their parent company, Cineworld. Cineworld has been contacted by the Guardian for comment.

The actor Andrew Garfield came out in support of the workers on Wednesday morning, telling Sky News: “It’s awful. It’s indicative of every aspect of our culture now, this massive social divide.”

The London living wage is calculated by an independent body, the Living Wage Foundation, and is £9.75 an hour. Picturehouse points out that Bectu agreed to a pay rise in September 2016 and claims that, once breaks were accounted for, front-of-house staff were paid more than the living wage.

Curzon, another cinema chain, agreed to pay its staff the independently calculated rate in 2014 after a similar dispute.