An investigation was under way among senior Labour Party officials today after protesters breached security to barrack Tony Blair from the floor of the party conference hall.

Demonstrators said they had been given passes to enter the Brighton Centre after applying to join the Labour Party just days before.

"There were no checks. We couldn't believe we'd got away with it," one said.

The two interruptions to Mr Blair's speech yesterday were amplified by a huge clash of pro and anti-hunting demonstrators who surrounded the venue. Riot police dodged eggs, fireworks and smoke-bombs as they struggled to keep the two factions apart. Four people were arrested during the noisy and tense demonstration.

But it was the ease with which protesters managed to enter the high-security island site which is troubling party officials.

The first heckling came just seconds into the speech when Hector Christie, millionaire grandson of Glyndebourne founder Sir John Christie, stood up and yelled: "You have blood on your hands."

Mr Blair was unfazed by the interruption saying from the podium that the heckler was lucky to be free to protest.

Around 20 minutes later there was a second commotion as about half a dozen hunting supporters stood up, began shouting and set off rape alarms on the balcony overlooking the conference room floor. Three people were arrested.

Sussex Police said they were satisfied the Prime Minister was never in danger and described the incident as a breach of individual trust, not conference security.

A spokesman said: "It was a breach of trust by those individuals, all of whom had visitor passes issued by the party.

"Internal stewarding in the conference hall is a matter for the Labour Party. They responded quickly to the disturbance and dealt with their visitors."

Millions of pounds have been spent on security at the Brighton gathering where delegates are only admitted with passes issued by the party.

A spokesman for the Labour Party said: "An investigation is ongoing."

A total of 13 people were arrested in connection with the day's events, including two men being questioned after the carcasses of a horse, cow and two calves were dumped in the city centre.