Prime Minister Tony Blair tonight told the British people he had ordered troops into action against Iraq to help rescue the world from "a new threat of disorder and chaos" posed by terrorist groups and dictatorial regimes with weapons of mass destruction.

"My judgment as prime minister is that this threat is real, growing and of an entirely different nature than any conventional threat to our security that Britain has faced before," Blair told the nation in a five-minute televised address, recorded before he headed to a European Union summit in Brussels.

The speech climaxed a day when Britain sent its forces into battle with a mixture of martial determination and civil unease, with the widespread feeling that the fate of its troops and its political leader rested in the hands of its superpower ally, the United States.

The House of Commons, which just two days earlier was the scene of a large-scale but unsuccessful revolt against military action, rallied around Blair, whose approval ratings have begun to climb again in public opinion polls after a long and steady decline. But antiwar protesters took to the streets of London and other major British cities, pledging to shut down traffic and businesses in an effort to force British withdrawal from the fighting.

The mood even among government leaders who have spearheaded involvement in the U.S.-led effort was somber. Foreign Secretary Jack Straw, emerging from this morning's cabinet session, told reporters he understood that Iraqi civilians would suffer during the conflict.

"There will be innocent civilians killed in this military action and we can't use euphemisms to cover up that reality," he said. "But what we say is that the number of Iraqi lives saved by this military action will far exceed the number of people who sadly will be killed.

"It is a terrible calculation to make, but it is one we have to make if there is to be proper justification for military action."

Popular ambivalence was reflected in a controversy over whether British leaders had been consulted in advance about the first U.S. bombing attacks on Baghdad early this morning. After telling reporters that no action was expected overnight, Downing Street officials were surprised to learn of the U.S. assault. They said the prime minister was awakened by a phone call from President Bush at 2 a.m. local time with word the assault would take place.

An unidentified military source told the Reuters news agency that the United States had launched the first strike without British involvement and that the British Defense Ministry was "in effect, not part of the planning." But Defense Secretary Geoff Hoon later insisted that he had been aware of the assault well in advance.

The controversy underscored anxiety here about the role of the 40,000 British troops who have been dispatched to the Persian Gulf region. Opinion polls have consistently shown most Britons opposed to military action without U.N. approval, and Blair acknowledged tonight that there were "deep divisions of opinion in our country." But he pleaded for national unity, saying the choice was clear in dealing with Saddam Hussein, the Iraqi president: "Back down and leave him hugely strengthened, or proceed to disarm him by force."

Despite public misgivings, Hoon was greeted with solid support when he appeared before the House of Commons this afternoon. "Whatever were our anxieties about the justification for war," Charles Kennedy, leader of the antiwar Liberal Democrats, told the House, "now that military action in Iraq has begun, we all hope and pray that it can be concluded as swiftly as possible with minimal casualties."

Outside Parliament, however, hundreds of protesters -- many of them schoolchildren -- chanted antiwar slogans and shouted "Shame on you!" at officials and the police. Some sought to block traffic and clashed with police. About a dozen were arrested. Thousands more gathered this evening in London and in major cities throughout Britain.

"Bush and Blair should be bombing Israel instead of Iraq," said Firdous Zyed, 18, a high school student from west London and a Muslim, whose family comes from India. "They are going to kill many innocent people."

Other protesters were more equivocal. "I think there's a certain ambivalence on both sides," said John Townsend, a retired education consultant from north London. "I know no one who wants to see Saddam Hussein remain in power. But that doesn't lessen my objection to the way this is being done, the damage to international institutions and to the reputations of the U.S. and Britain."

He added: "I think the protests have raised public consciousness about what's involved in all this and forced our political and military leaders to be far more careful about their targets. It's a very good reason to keep on protesting."

Muslim and Anglican clerics denounced the war. The Muslim Council of Britain called the start of war "a black day in our history." Four Anglican bishops in a joint statement said, "We have questioned whether this military action is justified, and history alone will reveal the truth."

Among Britain's rowdy tabloids there were few such reservations. Many led their front pages with the stirring address that Lt. Col. Tim Collins, leader of the 1st Battalion of the Royal Irish Regiment, gave his 800 troops in Kuwait, about 20 miles from the Iraqi border. "The enemy should be in no doubt that we are his nemesis and that we are bringing about his rightful destruction," he was quoted as saying. Hussein "and his forces will be destroyed by this coalition for what they have done. As they die they will know their deeds have brought them to this place. Show them no pity."

At the same time, Collins cautioned his troops not to kill Iraqis who throw down their weapons. "If someone surrenders to you, then remember they have that right in international law and ensure that one day they go home to their family," he said. "The ones who wish to fight, well, we aim to please."

"My judgment as prime minister is that this threat is real, growing and of an entirely different nature than any conventional threat," Prime Minister Blair told the nation.Pro-war student Richard Evans, right, watches marchers along Oxford Road in Manchester. They were among thousands of antiwar protesters who gathered in cities throughout Britain.