Anti-war campaigners are urging people to walk out of work, school and college on the day war starts with Iraq.
Protesters stage a "die in" demonstration in Whitehall on Monday
The Stop The War Coalition wants protesters to mass outside the Houses of Parliament if Iraq is attacked and say there is a "clamour" for such action.
A group of demonstrators smeared in fake blood forced police to close the southbound side of Whitehall when they staged a sit-down to coincide with an emergency Cabinet meeting.
Monday has also seen more demonstrations at military depots in the UK as the prospect of military action draws closer.
The anti-war movement is this Saturday organising a repeat of the peace rally last month in London which attracted a crowd estimated to be more than one million strong.
As well as opposing the war, protesters will march under slogans calling for Tony Blair to resign as prime minister if conflict is already under way.
Andrew Murray, chairman of the Stop the War Coalition, said: "I believe he has forfeited the right to lead a democratic country by taking us into a war that people do not support."
At a news conference on Monday, Mr Murray said war now looked imminent and he argued the last ditch efforts at the United Nations were just a "cover" for the UK Government.
He said: "This is an unjustifiable war which is opposed by the great majority of the British people...
"Tony Blair has had a year to make the case to the British people about this war being necessary and he has failed."
The coalition wants a "national day of action" if war breaks out, with people across the country taking whatever action they could.
"This should and could involve people walking out from work, walking out from school, walking out from college, or at the very least having lunchtime meetings," said Mr Murray.
The TUC is holding a special meeting on Wednesday to discuss its stance on military action.
There is a clamour among ordinary civil servants for us to take full part in
any protest should war start
Public and Commercial Services Union
Mr Murray said the law prevented unions from calling for national walk-outs in protest.
But he urged the TUC to make clear it would give protection to anybody facing action from their employers for peaceful anti-war protests.
Keith Crane, a local official of the Public and Commercial Services Union, predicted many Whitehall civil servants would take part in lunchtime protests in Westminster.
"I have never known a time that is so charged within the civil service," said Mr Crane.
He listed the Office of the Deputy Prime Minister, the Department of Education and Skills, the Home Office, the Department of Environment, Food and Rural Affairs, and the Inland Revenue among the places where staff would protest.
On Monday, peace protestors set up camp at a military depot in Berkshire in an attempt to stop bombs being loaded on to United States Air Force B52 bomber planes.
Around 16 demonstrators are attempting to stop the movement of munitions from an underground storage facility to RAF Fairford in Gloucestershire.
At Fairford itself, around 50 elderly women from the Grannies For Peace group, have gathered.
A series of 'die-ins' were held across the UK on Monday as the prospect of war drew nearer.
Anti-war protesters pretended to die in at least 15 towns and cities, including London, Cambridge, Sheffield and Oxford.
On Saturday, thousands of protesters staged demonstrations in cities across the UK, including York, London, Portsmouth, Leeds, Exeter and Newcastle.