Tony Blair's speech to the Confederation of British Industry, in Islington, north London, had been trailed as possibly heralding a new nuclear era... but Greenpeace had other ideas.
Two protesters, one of whom said he was a trained climber, scaled the ceiling of the Business Design Centre and demanded the opportunity to give a 10-minute speech.
But CBI director general Digby Jones was in no mood to welcome gatecrashers, and said he did not give in to "ultimatums" before moving the entire event to a side hall.
Interrupted by clattering microphones and mobile phones in the packed room, Mr Blair said: "I'm going to give this speech if it's the last thing I do."
The protesters were arrested and taken to a London police station, but Greenpeace said the demonstration "was the only way we could say what we wanted to say".