The Metropolitan police are facing legal action after officers tried to break up a picket outside a London medical school and arrested a trade union lawyer as he questioned whether or not they were acting legally.

Franck Magennis, a barrister from Garden Court chambers, who is seconded to UVW, an independent grassroots union that supports low-paid workers, has instructed lawyers to take action against the Met for false imprisonment.

In what he described as a piece of theatre intended to intimidate strikers, Magennis was arrested, handcuffed, then de-arrested by police outside St George’s hospital, University of London, during a strike by outsourced security guards.

Alongside his personal legal action, Magennis said that the UVW, where he is head of the legal department, could take action against the Met claiming the police infringed its members’ rights to take industrial action.

Susie Labinjoh of Hodge Jones & Allen solicitors, who is representing Magennis, said: “Magennis’s arrest clearly raises important constitutional issues. We will be looking at all legal avenues to ensure that the police are held to account, that trade union members are not criminalised for going on strike, and that people are not arbitrarily arrested. The police must respect and uphold union members’ right to protest.”

I am back on the picket line where yesterday I was unlawfully arrested. If @StGeorgesUni think they can use the police to intimidate striking workers, they have another thing coming. I've instructed @HJAcivillibs and am suing the police, donating all damages to the strike fund. pic.twitter.com/7OGC0ZYnkP — Franck Magennis (@FranckMagennis) January 14, 2020

Outsourced security guards at St George’s medical school in Tooting, south London, have been taking industrial action this week because they want to be taken in house by the college and given the same terms and conditions as directly employed workers.

Unlike in-house workers, the security guards, who are employed by Noonan services, part of the multinational outsourcing conglomerate Bidvest, receive just the statutory minimum sick pay, annual and parental leave, and pension contributions, UVW said.

A dozen police officers confronted the workers on their picket line soon after they began their strike on Monday morning, claiming the demonstration was an illegal nuisance on NHS property and issuing strikers with a letter from the university demanding that they move their picket.

It was when Magennis inquired into the legal basis of the warning that he was arrested. He was released five minutes later on the condition that he left the area immediately. The picket was subsequently broken up.

“It’s my view that the police were clearly attempting to carry out a piece of theatre where they arrested a lawyer who was carrying out a conversation with a police officer to intimidate other people on the picket line,” Magennis said. “If my false imprisonment goes unchallenged the Metropolitan police would in effect be allowed to criminalise what is lawful civil activity. This would have a chilling effect on workers’ rights.”

The Guardian has contacted Noonan services and the Met for comment.