Tony Blair performed an abrupt change of tack by claiming that overthrowing Saddam Hussein by force would be an act of humanity.
As millions of anti-war protesters took to the streets of London, Glasgow and other British cities, the Prime Minister departed from his usual argument that war is a last resort to protect the West against Iraq's illegal weaponry. Instead, he put what he called a "progressive" case for bringing down Baghdad's hated dictator.
"This is a regime that contravenes every single principle or value that decent people and people in our movement should believe in," he told delegates to Labour's spring conference.
"If the result of peace is Saddam staying in power, not disarmed, then I tell you there are consequences paid in blood for that decision too. These victims will never be seen. They will never feature on our TV screens or inspire millions to take to the streets. But they will exist nonetheless.
"Ridding the world of Saddam would be an act of humanity. It is leaving him there that is inhumane."
But only minutes before he made this uncompromising call for Saddam Hussein's overthrow, Mr Blair also indicated that the Iraqi president could still save himself and his government by agreeing to co-operate unconditionally with UN weapons inspectors.
In his first public reaction to Friday's UN Security Council meeting in New York, Mr Blair agreed that the inspectors should go back to Iraq to continue their mission to disarm the regime, and report to the UN again on 28 February.
Jack Straw, the Foreign Secretary, flew back from New York early yesterday to spend the day on the telephone to his counterparts around the world, trying to win them over to the British view that Saddam Hussein is still indulging in his 12-year game of evading UN controls. Foreign Office staff are now working on the text of a UN resolution which would condemn Iraq for failing to comply with resolution 1441, passed by the Security Council last November, and pave the way for a military strike.
Once the US and British have agreed a draft, they will begin negotiations with other permanent members of the Security Council to try to achieve unanimity. But their prospects of winning over France, Russia and China appear to have receded after Friday's session. This leaves Mr Blair with the nightmarish prospect of sending British troops into action without direct UN sanction, risking a rift in the Labour Party and more demonstrations to match yesterday's vast display of opposition.
The Prime Minister was heard in respectful silence by 2,000 Labour delegates. Some hailed it as the best speech of his career. During the standing ovation that followed, just three people held up banners saying "No blood for oil".
Mr Blair said the reason he felt strongly about tackling Saddam Hussein, as a Labour leader more than as a Prime Minister, was the "moral case for removing Saddam".
"It is not the reason we act. That must be according to the United Nations mandate on weapons of mass destruction. But it is the reason, frankly, why if we do act, we should do so with a clear conscience," he said. "Yes, there are consequences of war. If we remove Saddam by force, people will die and some will be innocent. And we must live with the consequences of our actions, even the unintended ones. But there are also consequences of 'stop the war'."
In a direct message to the millions of protesters marching around the world – some 25,000 just yards from where he spoke inside the heavily policed Scottish Exhibition Conference Centre – Mr Blair read out letters from Iraqi exiles detailing torture and political oppression under Saddam Hussein.
The Prime Minister appeared to accept his political career rested on this issue. He concluded his speech by saying: "It is indeed the testing time, the tough time, but if we come through it I tell you the price is not just a government able to carry on. It is far more important than that: it is a signal that we will have changed politics for good."
GMB general secretary John Edmonds said afterwards: "If the argument is so strong, why do it alone? If the argument is so strong, why not carry the world with us? Those were the questions that were not answered."
Earlier David Blunkett, the Home Secretary, became the first Cabinet minister to admit that party members had left the Labour Party in protest at the Government's stance on Iraq.
He said he had a message for "those party members who are so exercised now and those who have toyed with or left the party" that they had all joined the Labour "family" to change the world. And that is what they should continue to try to do.
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies