Tony Blair has urged the British people to put divisions over war with Iraq behind them and unite behind the UK's armed forces as they go into battle to remove Saddam Hussein.
Mr Blair takes centre stage in a London pub
The prime minister's message to the nation came as British forces were engaged in a US-led assault on southern Iraq from air, land and sea.
Royal Marine Commandos launched an aerial and amphibious assault on strategically important 'Red Beach', designed to provide a foothold for an assault on Baghdad.
Defence Secretary Geoff Hoon told BBC Five Live: "We are seeing significant signs of a breakdown in the organisation of Iraqi leadership elements at the centre.
"On the other hand, we are still seeing resistance in the south. It is a mixed picture."
Downing Street says Mr Blair was woken in Brussels to be told about the first news of British casualties.
A British military spokesman said eight Royal Marines from 3 Commando Brigade were killed when a US helicopter crashed in Kuwait.
Foreign Secretary Jack Straw told BBC Radio 4's Today programme: "Those service people who lost their lives and their loved ones are obviously uppermost in our hearts at the moment.
"The face that such risks have to be taken and it is a difficult environment even in terms of accidents illustrates the bravery and commitment of our troops."
Mr Straw, who has warned there will be civilian casualties, said he hoped for a short war.
But he added: "It's the early stages of the military campaign, so it's
hard to say how long it will take."
Conservative leader Iain Duncan Smith also expressed sympathy to the families of the dead troops.
But he added: "There will be casualties because that is in the nature of war."
Earlier, Mr Blair's televised plea for unity came as thousands took to the streets across the country to voice their opposition to the military onslaught against Iraq.
British artillery units have shelled Iraqi positions, after around 10 missiles were fired into Kuwait on Thursday.
Mr Blair praised the British armed forces.
"As so often before on the courage and determination of British men and women serving our country the fate of many nations rests," he said.
He said Saddam Hussein's consistent refusal to disarm left no choice but to go to war.
Mr Blair had a special message for the Iraqi people, stressing the UK's commitment to their country after the war would be "total".
"Our enemy is not you but your barbarous rulers," he said.
Mr Blair said he feared the twin threats of "brutal states" armed with weapons of mass destruction and terrorist groups could bring catastrophe.
"My judgement as prime minister is that this threat is real, growing and of an entirely different nature to any conventional threat to our security that Britain has faced before," he said.
He added: "I know that this course of action has produced deep divisions of opinion in our country.
"But I know also the British people will now be united in sending our armed forces our thoughts and prayers - they are the finest in the world and their families and all of Britain can have great pride in them."
The prime minister stressed his and President George Bush's commitment to restarting the Middle East peace process.
In what will be seen as a bid to silence critics of the US, he said revenue from Iraq's oil would be put in a UN trust fund for Iraq's benefit only.
Earlier, Foreign Secretary Jack Straw tried to steel people for civilian deaths in the conflict and Defence Secretary Geoff Hoon warned that the war might take time.
On Thursday, RAF Tornados flew a series of missions over Iraq, dropping laser-guided bombs on military targets.
The Marines' assault on Red Beach - at the head of the Persian Gulf on the strategically important Al Faw peninsula - took place less than an hour after a massive artillery bombardment on southern Iraq.
The Queen sent a message of support to troops in the Gulf. Members of the Royal Family are planning to visit military bases in the UK.