The government will face co-ordinated industrial action and civil disobedience once the true scale of its budget cuts becomes clear, union leaders warned today as they claimed 150,000 public sector job losses are already in the pipeline.

Police forces, councils, courts and hospital trusts are laying off workers even before the bulk of the spending cuts are announced in October's comprehensive spending review, research by the GMB union suggests.

And in a sign that mainstream Labour's attitude towards the cuts is hardening, Harriet Harman, the deputy leader, said the party felt "militant" against extreme cuts. She defended unions' right to strike, though she insisted that nobody wanted to see people's lives disrupted.

Union leaders at the TUC conference, which opens tomorrow, were tonight talking about the inevitability of industrial action after next month's spending review, when the scale of the coalition's cuts will become clear. The unions will debate plans that would see them mandated to "support and co-ordinate campaigning and joint union industrial action" to fight the cuts, under pressure from the more radical unions to demonstrate that they will stand up to the government.

Union leaders in Manchester had hoped to avoid talk of a winter of discontent in order to gain public support once the cuts begin to bite into people's services, welfare payments and housing bills in the spring. But tonight the TUC was struggling to contain anger over the cuts, with some unions pushing to act sooner to see off job losses and changes to their members' pensions.

Mark Serwotka, general secretary of the Public and Commercial Services Union, said industrial action was inevitable, adding that unless unions fought back together the future would be "bleak". "Over 100,000 civil service jobs have been cut over the past six years and we are now being hit by closures and cuts even as the sword of chancellor George Osborne hangs in the air.

"We ain't seen nothing yet. People are very worried and demoralised and are just waiting for things to get worse."

Bob Crow, general secretary of the Rail Maritime and Transport union, said a campaign of civil disobedience was needed to fight spending cuts.

"Maybe we need Batman climbing up 10 Downing Street, Spider-Man on Buckingham Palace as part of peaceful demonstrations of civil disobedience. This is an opportunity for the entire trade union movement to come together and mobilise support. Unions should also link up together, because we are confronting the same enemy. Otherwise they will be picked off one at a time."

However, Les Bayliss, who is hoping to be general secretary of Unite, said public sector strikes would only deprive vulnerable people of services they needed, and the ones the Tories wanted to cut.

Bayliss said: "Strikes will also change the victims – our members – into the villains of the piece. The story will get changed from government savagery to union militancy."

The GMB research, a survey of its officers to document the number of jobs they are defending up and down the country, identified 19,198 jobs under threat in Scotland, 8,680 in the south-east, 8,604 in the West Midlands and 8,176 jobs in the south-west region. NHS trusts are planning 36,000 job losses, the courts 15,000, and individual councils and police forces have also announced waves of redundancies.

Today, leaked documents suggesting that the coalition has considered another £2.5bn in benefit cuts drew further fury from charities. But the letters, from June, were played down by the government, which said the plans had moved on.

Paul Kenny, the GMB general secretary, said: "Current job losses already announced in the public sector of nearly 150,000 are just the top of the iceberg heading for our services and our economy when the comprehensive spending review finally hits home next month. Unemployment and cuts in public services follow the appointment of a Tory-led government like night follows day.

"The ideology of the Tory party is for a smaller state and they are hellbent on using the recession to impose these needless and ideologically driven cuts in public spending."

Harman told the BBC: "Well, we feel very concerned indeed, yes, about threats to jobs, and we don't accept the argument that somehow this is entirely necessary to cut the deficit at this speed. We think it's actually a threat to the economy. And the arguments that the big society can take the place of public services we think are disingenuous. So to that extent, yes, we do feel militant about it. We're concerned about the effect on people."

She said that no one wanted to see strike action, but unions had the right to strike. She added: "But as far as actual public services are concerned, I think we will see trade unions campaigning alongside local communities when vital public services are threatened."

Tonight the government would not confirm whether Osborne and David Cameron would be meeting union leaders before the spending review announcement in October.

A spokesman said: "We're not seeking confrontation, we're working constructively and we've consulted widely with public-sector frontline workers whose work we value. We think often union leadership isn't always representative of its entire membership. There's significant public understanding of what we're trying to do in the spending review."