In the first major protest in the United Kingdom since the war officially ended, demonstrators - 20,000 by official estimates - vented their ire at the invasion and the reasons given for it.
Coordinated global protests, from Seoul to Spain and San Francisco, coincided with the London-march, with demonstrators across continents calling for an end to the US-led occupation of Iraq.
The protesters also demanded an end to Israel's continued occupation of Palestinian territory.
Sweeping through the streets of London, the marchers served noisy notice that Prime Minister Tony Blair faced a crisis of credibility.
"It was all lies," protester Peter Mason said, referring to the coalition's pre-war hype about Iraqi weapons of mass destruction and its inability to discover any after the conflict ended.
"The millions who demonstrated before the war were right," insisted Mason.
In February, around a million people had marched through London trying to prevent the war in the biggest political protest march in British history.
Organisers of Saturday's protest said they planned more rallies when US President George Bush visits UK in November.
London Mayor Ken Livingstone told a packed Trafalgar Square the way to secure peace in the Middles East was to "get out, get out, get out."
Hundreds of protesters also took to the streets in Spain, whose pro-war government was an important ally of Bush.
In Seoul, over 2,000 people demonstrated in the largest rally against a US request for South Korean combat troops for Iraq.
Echoes of protests were also heard in Turkey, where around 5,000 people marched in Istanbul and capital Ankara.
Witnesses said at least one person was detained in Istanbul where the crowd burnt US and Israeli flags.
In France, some 3,000 protesters took to the streets while a motley crowd gathered in Greek capital Athens, denouncing the war.
"Stop imperialist intervention" and "Occupiers out of Iraq," said the banners held aloft by protesters.