The playwright Harold Pinter told an anti-war demonstration that he was ashamed to be British because of Nato's bombing of Yugoslavia.

At the gathering of more than 6,000 marchers in a park next to the Imperial War Museum in London on Saturday, he described the current peace talks as a sham, and claimed that the war had been totally unwarranted.

Standing in the shadow of the two 15in naval guns that sit at the entrance to the museum, the playwright threw his words out like stones, each of them aimed at the Labour government.

"I am sure those people here today who voted the Labour party into power share the same feeling Ð a deep sense of shame, the shame of being British.

"Little did we think two years ago that we had elected a government which would take a leading role in what is essentially a criminal act, showing total contempt for the United Nations and international law."

Pinter said Britain's leaders had been engaging in despicable hypocrisy, and contrasted Tony Blair's description of the nail bombing of a bar in Soho's Old Compton Street as "barbaric" with his defence of the cluster bombs dropped on Yugoslavia as "civilisation against barbarism".

"These cluster bombs cut children to pieces and this is an act which takes place 15,000ft under those brave bombers. An act which Mr Blair, with his moralistic Christianity, applauds," Pinter said.

"Let us face the truth. The truth is that neither Clinton nor Blair gives a damn about the Kosovar Albanians. This action has been yet another blatant and brutal assertion of US power using Nato as its missile. It set out to consolidate one thing Ð American domination of Europe. This must be fully recognised and it must be resisted."

The march, which began at the Embankment, had been organised by the Committee for Peace in the Balkans before the peace talks began. The organisers went ahead with the protest because they said it was "obscene" that the bombing was continuing.

Carrying anti-Nato flags, target placards and crosses with the name of the civilian dead in Serbia and Kosovo, as well as trade union banners, the marchers chanted anti-war slogans and demanded that money be spent on welfare rather than warfare.

The march coincided with a number of other anti-war marches around the world, including one outside the Pentagon in Washington, which sent messages of support.

A rally in Glasgow was addressed by the Labour MP for Linlithgow, Tam Dalyell. Alice Mahon, the Labour MP for Halifax, told the London crowd that the real US objective in the war was the occupation of Yugoslavia, a country which had "resisted 72 days of criminal bombardment".

She said that what she had learned was that Nato could now destroy any country from 10,000ft in the air and the only way smaller countries could defend themselves would be to obtain nuclear weapons.

"The Committee for Peace in the Balkans intends to continue its work when the bombing has stopped," she said. "We are not going to stop until justice is done."

¥ About 200 Kosovo Albanians gathered in Trafalgar Square in London yesterday at a rally to give thanks to Nato and the British government. Waving British and European Union flags as well as those of the United States and the Kosovo Liberation Army, they marked a minute's silence for the "Albanian heroes" of the war. Chanting "Free Kosovo", they demanded that no Russian troops or Serbian police should be in Kosovo.

Ben Bradshaw, the Labour MP for Exeter, said that while he wanted to say "I told you so", it was not a day for gloating because there was a lot of work to be done.

"We can show an example to the rest of the Balkans how people can live together," he said.