Lindsey German, convenor of the Stop the War Coalition, said Sir John Chilcot’s report underlined everything the group had been saying for more than a decade.
She said: “We have been saying for years that Blair was disregarding the UN and was in this hideous relationship with George Bush.
“Chilcot should not be the end of the matter - it must be the beginning of legal and political action against Blair.”
John Sauven, executive director of Greenpeace, said: “This report confirms what we already knew - that the Blair government led Britain into a disastrous war on a drumbeat of exaggerated threats, guesswork dressed as intelligence, and windy rhetoric.
“The formidable challenges humanity is facing today require international co-operation more than ever before. We cannot have a healthy environment without peace, and we cannot have peace without a healthy environment. This is why Greenpeace will keep campaigning for both.”
Kate Hudson, general secretary of CND, said the report was a “damning indictment” of the conduct of Mr Blair.
She told said: “All the big questions about lies and the legality of war have now been answered. It is clear that Blair was a prime minister with very little concept of democracy and little respect for international law or the United Nations.
“He seems to have pursued his own path, with the United States, completely outside the normal legal or political process.”
Mark Serwotka, leader of the Public and Commercial Services union, said: “We took absolutely no pleasure in being proved right about Iraq a decade ago and we take no pleasure in it now, but those responsible for what was a horrendous crime must be held to account.
“We were lied to, not only about weapons of mass destruction. We were also promised a swift resolution and a commitment to building a safe, secure nation, but instead we’ve seen a country ravaged by years of civil war, sectarian violence and terrorism.”
The Liberal Democrats opposed the military intervention in 2003 under the leadership of Charles Kennedy.
Current leader Tim Farron said: “Blair was fixated in joining Bush in going to war in Iraq regardless of the evidence, the legality or the serious potential consequences.”
He said Islamic State - also known as Daesh - had taken hold because of the absence of post-conflict planning.
Mr Farron said: “Charles Kennedy’s judgment has been vindicated in every respect. I hope those in the Labour and Conservative parties who were so forceful in their criticism of him and the Liberal Democrats at the time are equally forceful in their acknowledgements today that he was right.”
He added: “I hope Sir John Chilcot’s findings can in some way provide comfort to the families of the British servicemen and servicewomen who lost their lives, and to the people of Iraq. Hundreds of thousands of Iraqis have lost their lives and a generation has been shaped by this illegal war.
“Far from being Bush’s passenger, Blair was his co-pilot in taking this catastrophic decision which has destabilised Iraq, provided the hotbed for Daesh and tarnished Britain’s reputation around the word.”
Tory MP James Heappey, who served in the Army in Iraq, tweeted a photo of the inquiry with the caption: “So why was I really in Basra?”
Plaid Cymru MP Jonathan Edwards claimed Labour MPs’ attempts to unseat anti-war leader Jeremy Corbyn are linked to the report, tweeting: “The reason for the Labour coup v Corbyn now clear #Chilcot”.
Green MP Caroline Lucas said: “Chilcot’s report is damning for Blair, his cabinet and all those MPs who voted to take this country into an illegal and immoral war in Iraq. Iraqis continue to pay the price for an invasion that took place long before other options for a peaceful resolution were explored.
“This report confirms the series of serious failures that led to this disastrous war. We know for sure that government ministers, including Tony Blair, lied to the public about their reasons for going to war. He said he would support George Bush ‘whatever’ eight months before the war - and thousands of lives were lost because he stuck to that promise despite the evidence in front of him.”
Senior Labour MP Frank Field, who voted for the war, said: “What is now clear is the total incompetence of Tony Blair in launching a war and having no plan for the day after the Iraqi regime was overthrown.
“That gigantic political error resulted in the deaths of tens of thousands of Iraqis, as well as 179 British soldiers.
“The Middle East has been thrown into chaos.
“Tony Blair maintains he has nothing to apologise for.
“If this record is not one which warrants an apology, it is difficult to think what is.”
Ms Lucas also called on MPs who voted for the war, including Prime Minister David Cameron, to apologise.
The Brighton Pavilion MP said: “There’s no doubt that Tony Blair should take much of the responsibility for this disaster - but every MP who closed their ears and eyes to the facts and voted for the war should now publicly apologise.
“Four hundred and 11 MPs walked through the lobbies to vote alongside Blair for the Iraq War - and both parties need to take responsibility for that. The Prime Minister is the only leader in Westminster to have voted for the war and he should apologise in full for doing so.”
Amnesty International UK director Kate Allen said: “It’s a tragedy that politicians and their advisers failed to properly assess the human rights consequences of such a massive military operation, including the horrible sectarian violence it helped unleash, and it’s also a tragedy that the horrors of Abu Ghraib and cases like Baha Mousa all followed.
“Hundreds of thousands of people died in Iraq, during the invasion and its extended aftermath, including UK service personnel. It’s therefore vital that lessons are learnt after Sir John Chilcot has so comprehensively pointed towards what some of those are.
“One way of showing that the Government has tried to learn lessons from Iraq would be for it to ensure that all credible allegations of unlawful killing, torture and unauthorised detention at the hands of the UK armed forces in Iraq are properly investigated.”
Unite union general secretary Len McCluskey said: “Chilcot confirms what millions of us knew in 2003 - the case for war had not been made. It was an unnecessary conflict, launched on the basis of flawed intelligence, secret diplomacy and with no sound legal basis.
“It has cost hundreds of thousands of lives and made both the Middle East and the wider world less secure.
“Today our thoughts must be with those who lost loved ones, and with the people now living in the wretched insecurity that followed this war, but it is long past time that those responsible were held to account.”