Hundreds of demonstrators joined marches and rallies across Britain on Saturday as part of a "national weekend of action" against a proposed new law that would give police extra powers to curb protests.

The Police, Crime, Sentencing and Courts bill would toughen measures officers can take to disperse demonstrations, such as imposing time and noise limits, which campaigners and activists fear would be used to curb dissent.

Since the bill was brought before parliament last month, there have been sporadic protests, notably in Bristol, southwest England, where demonstrations turned violent with officers and a police station bombarded with bricks and glass bottles and police vehicles set on fire.

Prime Minister Boris Johnson criticised what he described as "disgraceful attacks" on officers, but protesters have accused police of using heavy-handed tactics.

On Saturday, climate change group Extinction Rebellion (XR) and the Black Lives Matter (BLM) movement joined other activists for "kill the bill" rallies in London and other towns and cities, including Manchester, Sheffield, Leeds and Brighton.

"The government are trying to curtail protests - especially BLM and XR - that is what this bill is all about. We want the clauses in this bill about protests quashed," said Mark Duncan, one of a crowd of more than 500 marching through central London, banging drums and chanting.

Days of protests by Extinction Rebellion paralysed parts of London in early 2019, action which fuelled calls from some politicians for the police to be given the tougher powers to prevent excessive disruption.

Demonstrations had not been permitted while a coronavirus lockdown was in place, but restrictions were eased this week, meaning organised rallies can go ahead providing they are "COVID secure".

In London, police warned, "enforcement action will be taken, if needed, in the interests of public health".

Some senior officers have said the "kill the bill" tag was deliberately provocative as "the bill" is a nickname in Britain for the police.