Deputy British Prime Minister Mr John Prescott is alleged to have thrown a punch at a demonstrator in North Wales this evening.
Mr Prescott was said to have been manhandled by demonstrators before being escorted into the theatre by police.
A Labour Party spokesman said the deputy prime minister had "defended himself from the attack". It is understood one man is being questioned over the incident.
Eye-witness Mr Ron Ingram, a farm labourer, claimed he saw the deputy prime minister lash out when he was hit by an egg.
"Somebody threw an egg and Prescott just turned around and thumped them. I saw it. We were just protesting. The egg hit him and he hit out.
"I'd like to know why the lad's been arrested and not Prescott. There was pushing and shoving going on something terrible. It got quite nasty."
The deputy prime minister had been heckled earlier in the day when he was confronted by an angry ex-shipworker during an election campaign visit to Merseyside.
Prime Minister Mr Tony Blair was also heckled by a woman lambasting him over the state of the NHS.
But Conservative leader William Hague did not escape either. He was heckled during a walkabout in Wolverhampton.
PA