By Ben Davies BBC News Online political staff

Demonstrators shouted, chanted, whistled and - in one case - screamed their anger over the ongoing attack on Iraq.

Column after column walked by the Thames into Parliament Square and then on up Whitehall and past Downing Street.

In Piccadilly Circus they merged with the other part of the march which began over in Bloomsbury before continuing on to speeches in Hyde Park.

BBC News Online spoke to human rights activist Bianca Jagger as she marched along side union bosses and leaders of the Stop the War Coalition.

One protester screamed her opposition as she passed Downing Street

She said: "We are letting the prime minister know that we oppose his decision to go to war without the support of the people in this country, without the support of the United Nations, in contravention of international law.

"We are protesting his reckless disregard of the UN Charter, we are concerned about the loss of innocent life and we are concerned at the deaths of British servicemen."

Ms Jagger also said she was anxious about the plight of the Iraqi people once the war was over.

Many other marchers said that the fact battles were already raging across Iraq did not alter their opinion that this was not a just war.

Among the marchers was 14-year-old Holly Strauss who had travelled with members of her family from Nottingham.

'Power and oil'

"I don't believe this war is right. It's mainly about oil and innocent people are dying," she said.

Barry O'Donovan was marching carrying a friend's baby in a papoose.

The south Londoner also believed that the war was driven more by American economic and political ambition than anything else.

"Just because war has started my opposition hasn't altered. It's American aggression over oil, over world domination.

"I didn't vote for George Bush and I feel Britain is carrying out George Bush's policies."

In the spring sunshine there was sometimes a carnival atmosphere. Some protesters had brought drums and other instruments and were walking to the rhythm.

Ms Jagger was just one of thousands of marchers

Two men wearing green hats inscribed with the words 'eco-warrior' were strumming guitars.

Plenty of police were on hand but as with the last anti-war demonstration the mood was relaxed.

But the anger was tangible whether through chants or banners.

One disabled man amid the crowd, with a grim look, seemed determined to walk all the way to Hyde Park.

If nothing else Saturday's demonstration shows that Tony Blair will have another fight on his hands once war is over.

If his message to the British people on the outbreak war is anything to go by, the prime minister knows he now leads a country that is badly split.