The environmental campaigner George Monbiot last night tried and failed to make a citizen's arrest of the former Bush administration official John Bolton over alleged "war crimes" committed during the invasion of Iraq in 2003.

As Bolton, a former US ambassador to the UN, ended an hour-long discussion at the Hay festival, Monbiot, who had earlier challenged him for alleged breaches of the postwar Nuremberg Principles, defining war crimes, moved towards the stage waving a charge sheet. But security staff intervened and bundled Monbiot out of the tent as 20 supporters chanted "war criminal" and waved placards. The comedian Marcus Brigstocke, who tried to pursue Bolton as he left the other side of the tent, was also blocked by security staff.

When challenged by Monbiot during the debate to say why - in planning, preparing and waging war against Saddam Hussein - he was any different from Nazi war criminals condemned at Nuremberg, Bolton cited Iraq's defiance of the UN resolutions 687 and 678, which underpinned the 1991 Iraq war and ceasefire. That released other parties from the obligation to the ceasefire, he told Monbiot.

Earlier, Bolton had defended the US's right to launch pre-emptive nuclear attacks and to promote regime change or, if necessary, a military attack on Iran to prevent it acquiring nuclear weapons. As a lawyer, he said, he was not prepared to offer a view either on rendition or torture of suspects, because he had not studied the issues - a claim that provoked dismay.

Afterwards, Monbiot, a contributor to the Guardian, said: "I'm disappointed I couldn't reach him, but I made what I believe to be the first attempt ever to arrest one of the perpetrators of the Iraq war, and I would like to see that followed up."