The Stop the War Coalition is teaming up with the Muslim Association of Britain and the Campaign for Nuclear Disarmament to organise what it hopes will be its biggest demonstration yet.
The event will take place in London on 24 September, in response to what the three organisations perceive as the ongoing erosion of civil liberties - a trend aggravated by counter-terrorism measures and the new Serious Organised Crime and Police Act.
Chris Nineham, a spokesman for Stop the War, said: "Conditions are being put on the right to protest peacefully, which is something the anti-war movement should be concerned about.
"If we do not do something, protesters will be marginalised."
- See News analysis, page 10.