Up to 10 million people on five continents are expected to demonstrate against the probable war in Iraq on Saturday, in some of the largest peace marches ever known.

Yesterday, up to 400 cities in 60 countries, from Antarctica to Pacific islands, confirmed that peace rallies, vigils and marches would take place. Of all major countries, only China is absent from the growing list which includes more than 300 cities in Europe and north America, 50 in Asia and Latin America, 10 in Africa and 20 in Australia and Oceania.

Many countries will witness the largest demonstrations against war they have ever seen.

The majority will be small but 500,000 people are expected in London and Barcelona, and more than 100,000 in Rome, Paris, Berlin and other European capitals. In the US, organisers were yesterday anticipating 200,000 marching in New York if permission is given. A further 100,000 are expected to march in 140 other American cities.

What is extraordinary, say the organisers, is the depth and breadth of opposition that the US and Britain are meeting across the world before a war has even started.

"This is unprecedented. Demonstrations only got this large against the Vietnam war at the height of the conflict, years after it started," said a spokesman for Answer, a coalition of US peace groups which helped organise a march of 200,000 people last month in Washington.

Many in the global peace movement optimistically hope that public opposition to a war is becoming politically significant and could now affect the timing of an invasion of Iraq and possibly even help avert conflict altogether.

"The internationalism of the opposition is the most powerful weapon people have. It's all we have. We think that Bush and Blair are well aware that global opposition is mounting fast and that they are now desperate to start the war before they are completely isolated by world opinion," said a spokesman for United for Peace and Justice, a US coalition.

New polls in Europe and the US yesterday suggested that opposition is still mounting and is likely to continue even if the US gets a second resolution. Spanish and Dutch polls showed that more than 70% now oppose even UN-mandated action, with slightly fewer in Italy. Yesterday CND reported that it was struggling to cope with the deluge of people wanting to join.

In Germany, more than 300 towns are sending coaches to Berlin, where more than 100,000 people are expected to march.

"Opposition is broader than at any time in the past. This will be the largest peace march in 20 years," said Malte Keutzseldt of Attac, Germany. "The peace movement is getting older now, but a new generation of young people is deeply concerned. The churches and unions have linked to make the coalition far broader than even the anti-nuclear missile marches in the 1980s".

In Paris, a march organiser said that feeling was running high and that he expected the anti-war demonstration to be largest ever. The most unusual rally is expected to be in the international territory of Antarctica, where dozens of scientists and others at the US McMurdo base on the edge of the Ross sea will take to the ice.

The idea of an international day of action against the war was first suggested in London after the last peace march in October. It was discussed by peace and anti-globalisation groups from 11 countries at the European social forum in Florence in November, but only became truly international following meetings in Cairo, Egypt and Porto Alegre, Brazil, last month.

Since then the idea of coordinating international peace protests has spread rapidly across the world and up to 30 new cities a day are believed to be planning demonstrations. Next month activists from all continents will meet in London to propose further global actions.

Coordinated international demonstrations have flourished in the past five years with anti-capitalist marches and campaigns by environmentalists and anti-globalisers against corporations like McDonald's, Shell and Esso, and against global warming or international trade. Mostly organised on the web by activists working below the radar of the mainstream media, they have taken the establishment by surprise in many countries and only been reported by independent media.

"The whole world's marching," said Helmut, a German student in London. "This peace party should be better than the millennium celebrations."

· The Stop The War Coalition (STWC) is planning a display of mass direct action designed to bring Britain to a standstill on the day any war starts with Iraq. The protests would involve demonstrations in the centre of London and other big towns and cities, wildcat strikes by anti-war supporters and mass sit-ins at schools, colleges and universities across the country.

A spokeswoman for the SWTC said: "We do think there will be a whole wave of civil disobedience if war breaks out. People want to be peaceful and are quite slow to anger, but they will be very angry if after Saturday's mass show of opposition Tony Blair refuses to listen."