Workers across the UK will join May Day rallies amid warnings of "head on fights" in the coming months over spending cuts.
The Rail Maritime and Transport union urged workers to resist the looming "all out assault" after all the main parties warned they would cut spending after next week's election.
General secretary Bob Crow said: "We cannot afford to wait for politicians to unleash a £50 billion slash and burn attack on our public services, jobs and living standards after May 6.
"We have to start the fightback now and that means concerted action by trade unions to resist the all-out assault we know is coming."
The GMB said its members would be out in force over the weekend across the UK and Ireland, including a rally in London on Saturday, where officials from the union's sex workers branch will be among the speakers.