Speaking on the eve of the TUC congress, Unite leader Len McCluskey said no form of protest should be ruled out including “direct action”.
He urged a “campaign of resistance so that the government will take stock and perhaps take a step back” from their “attack” on workers’ jobs and pensions.
“I don’t think we can rule anything out,” he told the Andrew Marr Show.
The three-day TUC conference, which gets under way in London on Monday, is set to be the most highly-charged in recent years.
TUC general secretary Brendan Barber, who will address delegates on Monday, has already warned that Britain faced widespread strikes this autumn unless ministers changed direction on plans to raise public sector workers’ contributions to their pensions.
Mr McCluskey said the 1.5 million public sector workers in his union were in a “very angry mood” at what he called the coalition government’s “ideological” assault on their jobs and pensions.
These are tough discussions we are having but we have been making progress in these negotiations”
Danny AlexanderChief Secretary to the Treasury
He told the Andrew Marr Show: “The actions that will be taken will be widespread and I don’t think we can rule anything out.
“I noticed recently senior citizens protesting in Bristol by walking backwards and forwards across a zebra crossing and bringing things to a standstill.”
He also praised UK Uncut, which has targeted companies over alleged tax avoidance and staged sit-ins at banks, saying that that kind of “direct action” was what his members wanted, as well as traditional industrial action.
“They expect their leaders to give that type of leadership and to stand shoulder to shoulder with them when their terms and conditions are being attacked.”
Mr McCluskey also criticised Labour leader Ed Miliband over his decision not to back public sector strikes in June.
Mr Miliband said the 24-hour walkout by about 300,000 teachers and civil servants was “wrong” at a time when negotiations between the unions and government over pension changes was still ongoing.
But Mr McCluskey, whose union backed Mr Miliband for the Labour leadership and is one of the party’s biggest donors, said: “I think he made a fundamental error by attacking the strikes on 30 June, but he’s learning in his job.
“He’s got to be given time to construct his, hopefully, radical alternative and I hope that that will mean he understands he has to be on the side of ordinary working people.”
Chief Secretary to the Treasury Danny Alexander said he expected to “hear a bit of sabre rattling” during the TUC conference but he urged union leaders to consider strike action as “very much the last resort”.
The Lib Dem minister told Sky News reforming pensions was “in the long term interests of the country” and further walkouts would be “irresponsible at a time when talks are still going on”.
“These are tough discussions we are having but we have been making progress in these negotiations,” said Mr Alexander.
“I’m fully committed to these discussions… to try and make sure we can get to a position where we reach reforms everyone agrees are necessary.”