Jeremy Corbyn has apologised on behalf of the Labour party for the "catastrophic" occupation of Iraq. Speaking to activists after Sir John Chilcot published the eagerly awaited report into the invasion, the Labour leader condemned the decision to oust Saddam Hussein as the "most serious foreign policy calamity of the last 60 years".

Corbyn, who voted against the war and marched with more than one million peace campaigners before the invasion, blamed the occupation for tearing Iraq apart and spawning sectarianism.

"The war was not in any way as Sir John Chilcot says a 'last resort,'" he said. "It was an act of military aggression launched on a false pretext as the inquiry accepts and has long been regarded as illegal by the overwhelming weight of international legal opinion.

"It led to the deaths of hundreds of thousands of people in Iraq and the displacement of millions of refugees. It devastated Iraq's infrastructure and society.

"The US-British occupation led to a lethal sectarianism that turned into a civil war. Instead of protecting security at home or abroad, the war fuelled and spread terrorism across the region including in the countries that launched it."

Though he failed to mention Blair by name, Corbyn said MPs were "misled by a small number of leading figures". He apologised to the people of Iraq, which he said was still living with the consequences of the war and "the forces it unleashed" and the families of soldiers killed in Iraq who were in the middle of a conflict "they should never have been sent to".

Finally, he apologised to the British citizens "who feel our democracy was traduced" by the decision process including Blair's "I will be with you, whatever" memo to President George Bush eight months before the invasion.

The Chilcot Report concluded the UK – led by Tony Blair's Labour government – rushed into the conflict before it had exhausted all other options. He condemned flawed intelligence and said the government pushed the idea Saddam possessed weapons of mass destruction with "a certainty that was not justified."

Blair makes Westminster speech

His speech came after an emotional Blair made a 20-page speech in Westminster saying he would carry the decision to invade Iraq "for the rest of my days".

"The decision to go to war in Iraq, and remove Saddam Hussein from power, in a coalition of over 40 nations led by the USA, was the hardest, most momentous and agonising decision I took in my 10 years as British Prime Minister," he said. "For that decision, I accept responsibility in full – without exception or excuse."

Former foreign secretary Jack Straw, who was criticised in the report for a lack of post-war planning, said the decision was "a last resort" and with hindsight different decisions would have been made.