The Stop the War coalition said it expected workers to stage lunchtime protests and walkout for longer periods as well as join national demonstrations across the country when fighting begins.
The group announced it will stage another huge demonstration in London on Saturday when it expects people from across the country to demonstrate again against the war.
Peace protestors are also expected to congregate in town centres - and Parliament Square in London - on the first evening following the bombardment of Iraq.
The theme of the protest will be to "stop the war", but there will be a second message calling on Tony Blair to resign.
The TUC announced today it will be holding a special meeting of its general council on Wednesday to discuss the growing likelihood of military action.
The union organisation has already said it opposes military action without a second United Nations resolution.
Andrew Murray, chairman of the Stop The War coalition, told a news conference in London today: "It now seems clear that following yesterday's war party meeting, war is very imminent even though it is still opposed by the great majority of the British people.
"We feel there is no justification for the British government taking part in this war.
"Tony Blair has had a year to make the case and he has failed."
The group is calling on its supporters to demonstrate outside the House of Commons tomorrow and hopes that huge numbers of people will join Saturday's demonstration, which will follow a similar route to one held last month when more than a million people took to the streets of the capital.
Mr Murray said he believes people will walk out of work, schools and colleges the moment military action begins.
He said he hoped the TUC would give a lead on Wednesday and make it clear that unions will support any worker who takes action.
Keith Crane, a local official of the Public and Commercial Services Union, said: "I have never known a time that is so charged within the civil service.
"There is a clamour among ordinary civil servants for us to take full part in any protest should war start."
Mr Crane said he believed many civil servants who joined lunchtime protests will not return to work in the afternoon, adding: "The strength of feeling is incredibly high."
Departments where civil servants will take action include the Home Office, Inland Revenue and the Office of the Deputy Prime Minister.
