Anti-war supporters will stage a demonstration in Westminster as Sir John Chilcot presents his long-awaited report on the UK's role in the Iraq war.
Leaders of the Stop The War Coalition, CND and other groups will demand "truth and justice" and there will be calls for former Prime Minister Tony Blair and others to face the full force of the law.
Kate Hudson, CND general secretary, said: "The Iraq war was a disaster, a disaster that began with a lie.
"If Tony Blair and other politicians responsible had told the truth it would never have happened. A country was destroyed, millions of innocent Iraqis were killed, British soldiers were killed and terrorism has spread across the Middle East. Those responsible must now be brought to justice.
"We seek from the Chilcot Inquiry an accurate reckoning of the factors involved and finally to get clarity from the British state about this disastrous war. But it must not end there.
"The anti-war movement will gather in Westminster to demand truth and justice. It comes down to a principle - where individuals, no matter how lofty, are found to be responsible for crimes, they should face the full force of the law. No-one is exempt from justice."
Chris Nineham, of the Stop The War Coalition, said: "What the majority of people want from the next few days is an open admission that the war on Iraq was disastrous, illegal and wrong in itself, and that those who took us into it, led by Tony Blair, did so knowingly and by lying to people and parliament.
"Anything short of this will surely confirm people's suspicions that the Chilcot circus has been yet another convoluted attempt at a cover-up.
"Over the next few days, and for as long as it takes, Stop The War will be campaigning for the truth about Iraq to be publicly acknowledged.
"Many people also want to see that being a public figure and extremely rich doesn't exempt you from being held accountable when you commit crimes."