Green & Black Cross legal observers took the footage inside Fortnum & Mason and handed it to UK Uncut, which passed it on to the Guardian guardian.co.uk

Campaigners for the tax-avoidance protest group UK Uncut have claimed senior police officers "tricked" them into a mass arrest after a peaceful protest inside Fortnum & Mason's in London on Saturday.

Activists say they were given repeated assurances by a chief inspector from the Metropolitan police that they would be shown to safety after the protest, which she described as non-violent and sensible. However, when protesters left the luxury Piccadilly store on police instruction, they were kettled, handcuffed and taken into custody.

Their claims are backed up by footage, obtained by the Guardian, showing that, rather than being asked to leave, the protesters inside the luxury food retailer were told they were being kept inside for their own safety.

In the video, shot by observers for the legal volunteer group Green & Black Cross, a police officer can be seen telling protesters they would be directed towards "the safest parts" once they had left the building.

In all, 201 arrests were made during protests in London on Saturday, at which shops, banks and hotels were attacked by demonstrators who had broken away from the main, union-organised march down Whitehall to Hyde Park.

A total of 149 people have been charged with offences, including 138 charged with aggravated trespass in connection with the Fortnum & Mason protest.

The video also shows the officer agreeing with protesters that a breach of the peace had occurred outside the store, but not inside, and that Uncut protesters were being held inside so they did not become "wrapped up" in that disorder.

"As people leave, they're going to be asked to go left," she can be heard telling protesters. "They're just going into a safe environment because there's some disorder [outside] ... so we're trying to keep it sterile, safe, so people can get away to the tube station.

"People here are non-violent. It's sensible – we don't want them getting involved in stuff that makes it difficult for them," adds the police officer in the footage. Another officer also assured the protesters no one would be kettled if they left the building. A spokesman for the Metropolitan police said it would be inappropriate to discuss the matter while proceedings are active.

One Uncut activist, Jack Winstanley, who was held in a cell in Plaistow for 17 hours, said: "I feel totally betrayed by the police. They communicated the message fairly clear that we'd be released. And our protest had been totally peaceful. We then walked out into a kettle with police with riot helmets on. Not only could we not get home, we got arrested one by one."

Winstanley, a 26-year-old graphic designer from Brighton, said the Fortnum protest had been just like other UK Uncut protests. "It was creative and fun and exciting. There was a bit of cheering [and] whooping when people put up banners, and then things settled down and people were reading books and chatting and some people were talking to the staff about why we were there."

Several of those arrested said they had clothes and phones removed to be used as evidence.

Another Uncut activist, who was arrested for 20 hours and preferred to be known only as Jim, said: "There was this policewoman, and she spoke to the legal observers and said that all we had to do is turn left and we'd be free to go.

"But when we got out, they began arresting us. People in the crowd told a second female officer: 'You've lied to us. You said we could leave and now you're arresting everyone.' She replied: 'Yes, you're free to leave – to the police station. You're going to be arrested.'"

"Because we were at a peaceful protest, which had been the same as all UK Uncut actions to date, it really shocked me that they would arrest all 150 protesters. I feel tricked. Perhaps I shouldn't expect the police to tell me the truth," he said.

Also in the video is a 24-year-old female legal observer Jules Martin, who says customers in Fortnum & Mason are able to go about their business. Afterwards, she said: "The protesters never interfered with customers who wanted to browse. According to my notes, the cafe at the back was still open with waiters still moving about until after 5pm."