Protesters appeared to attempt to disrupt a visit to Parliament by Canadian prime minister Stephen Harper today.
Shortly before the premier addressed peers and MPs the doors slammed shut on the robing room venue and shouting was heard.
Guests, who did not want to be named, said the demonstrators were environmental activists who had spilt oil on the floor of the Royal Gallery outside.
Scotland Yard confirmed that three protesters were arrested following the security breach.
Campaigners against "tar sands", which produce more emissions than conventional oil, have posted a video and statement online about the incident.
The Love Canada Hate Tar Sands site stated: "We have entered Parliament to interrupt Harper's speech. We have managed to climb onto the roof with t-shirts saying 'oil out of politics', 'stop Harper' and 'stop the tar sands'. Two campaigners have spilled molasses on the floor outside of Parliament."