"Kill the Bill" protests denounce a contentious law currently making its way through Parliament which would grant law enforcement greater power to police or prevent protests.
Eighteen people were arrested in Manchester on Saturday for disrupting public transport during a protest against a new policing bill, British police said.
Greater Manchester Police described Saturday afternoon's protest as "largely peaceful" with the majority of the crowd dispersing by 16:00 local time.
But some of the remaining protesters "caused significant disruption to transport networks and members of the public" after seating on a tram line.
"I understand the desire to peacefully protest but by blocking trams this smaller group prevented passengers from travelling to work for essential journeys, including health and care workers," Chief Superintendent Andy Sidebotham said.
"This was clearly unacceptable and, in our duty to serve the public, we could not allow to be tolerated any longer," he added.
"Kill the Bill" protests were also held in other cities across the UK including Brighton, Cambridge, Nottingham and Sheffield despite a ban on large gatherings to prevent the spread of COVID-19. They denounce a contentious law currently making its way through Parliament which would grant law enforcement greater power to police or prevent protests.
Ten people were arrested a day earlier in Bristol after a group of protesters clashed with police.
Prime Minister Boris Johnson described the protesters as "a mob intent on violence and causing damage to property".
A journalist reporting on the protest said however that he was "assaulted" by police. A video he released on social media appeared to show an officer hitting him with a baton despite him clearly identifying himself as a member of the press.
"I was respectfully observing what was happening and posed no threat to any of the officers," he wrote.
The Avon and Somerset Police said in a statement that it is "aware of a video showing a journalist being confronted by officers".
"We're making efforts to contact him. A free press is a cornerstone of our democracy and we fully respect the media's vital role in reporting events fairly and accurately," it added.