Grumbles over Wednesday's scheduled appearance by John Bolton at the Hay festival turned caustic last night as the activist and journalist George Monbiot called for a citizen's arrest of the man George Bush controversially appointed to the UN in 2005. The Guardian columnist's demand - greeted with cheers from the audience - came just hours after an audience member challenged the veteran author Gore Vidal for appearing on the same festival bill as Bolton, who is widely viewed as a driving force behind the 2OO2 invasion of Iraq.

Monbiot expressed astonishment that a "war criminal" such as Bolton could "swim through the politest of polite soirees - which is of course Hay", without fear of proper interrogation. People such as Bolton and Tony Blair "would've been hanged" had a Nuremberg-like trial been held to investigate the war in Iraq, he added.

Speaking at a session to discuss the book Defeat: How They Lost in Iraq, with its author Jonathan Steele and former UN senior official Shashi Tharoor, Monbiot attacked the way Bolton and others had been "rehabilitated" after playing a role in the war which he characterised as "clearly a prima facie case of criminality".

Bolton, who was the US administration's undersecretary of state for arms control in the lead up to the iraq war, is due to appear with Nik Gowing at the literary festival, which is sposnored by the Guardian, on Wednesday evening.

Earlier in the day, the esteemed American man of letters Gore Vidal was asked from the audience whether he thought the invitation extended to Bolton, decribed by the questioner as a "war criminal", was justifiable. Vidal, deliberately or otherwise, appeared not to recognise Bolton's name towards the end of a wind-buffeted session.

It's not the first time a prominent neoconservative figure from the US has raised Hay hackles. At last year's festival, the Republican party stalwart Richard Perle faced flak from the audience as he defended his support for the invasion of Iraq.