Far-right activists have attacked trade union meetings and anti-racist groups in the past month in what campaigners and politicians say is an escalating campaign of intimidation and violence.

In the latest incident, a 20-strong group hurled concrete pillars, glass and rocks at a meeting on multiculturalism organised by Labour councillors in Barking, east London.

"It was terrifying," said Beverley, 48, an NHS worker who was hit by a rock, leaving her hand so badly damaged that she needed surgery and was on a drip in hospital for three days. "These people seem to think they can bully and intimidate people into staying away."

The attack last Thursday followed incidents in Liverpool, Brighton and east London involving people who claim to be supporters of the English Defence League (EDL), a far-right street movement. A spokesman for the group said the EDL was unaware of any of the incidents, adding that it did not condone violence.

The attacks follow disastrous election performances by the British National party, leading analysts to warn that some of its supporters may be turning their backs on electoral politics to focus on more violent street confrontations.

"The threat is that as far-right activists decide the electoral path is no longer possible ... we will see more aggressive street-based groups linking up and a rise in racially and politically motivated violence," said Nick Lowles from Searchlight.

Anti-racist campaigners said the attack in Barking appeared to be well organised and targeted. The group "appeared out of nowhere" as people were gathering in the foyer and had lookouts stationed outside as the assault continued.

"All of a sudden about 20 men and one woman came running across the car park screaming E-E-EDL," said Beverley, who did not want to give her second name in case of reprisals. "We rushed to lock the glass door ... but they didn't break stride, they just ran at it and smashed into it. They were head-butting it, kicking it, throwing things at it."

The reinforced glass came away in chunks, which were hurled at the unionists and anti-racist campaigners trapped in the foyer. Beverley said: "They were crazed on the other side of this glass wall ... They started ripping pieces of glass off and frisbeeing them at us through the holes and then they started hurling rocks at us."

George Barratt, a Labour councillor for Barking's Mayesbrook ward, who was due to speak at the meeting on multiculturalism and racism, said: "It is extremely disturbing. We don't want these thugs here and we won't tolerate them attacking our meetings."

In the run-up to the local elections Hope not Hate campaigners, who were on their way to deliver anti BNP leaflets in Essex, were confronted by around 40 or 50 far-right activists who refused to let them off the train at Grays in east London.

This month a radical bookshop in Liverpool was targeted by around 15 men claiming to be EDL supporters and last month in Brighton a meeting on multiculturalism was attacked by a group of 30 or 40.

Weyman Bennett, from Unite Against Fascism, said: "These attacks are escalating in their frequency and in the level of violence. They are now targeting trade unionists and elected councillors as well as anti-racists - it is classic fascist tactics and cannot be ignored."