Unite's Len McCluskey pledged to "fight all the way to the next election" as he said no form of protest, including the prospect of a general strike, should be ruled out.
He said the Government's policies, designed to balance the nation's books, were leaving working people feeling "battered".
Mr McCluskey said: "The reality is that this Government's policies are taking us on a path to poverty and we want to make certain we give people confidence throughout our nations to be able to stand up and resist.
"That's the only thing you can do, it's called democracy. The oldest form of democracy is protest, civil disobedience, any form of resistance that makes this Government take a step back and know there are millions and millions of ordinary working people in our nations who are not prepared to stand idly by and watch them destroy everything that we hold dear to us in our society."
He added: "When governments are acting in a way that is against ordinary, working people we have a right, in fact we have a duty, to stand up and protest."
Mr McCluskey, whose union is a major donor to Labour, said Ed Miliband's party had to show it was "on the side of the 99%, not the 1% who are ripping people off at every level".