Three environmental activists demonstrating during a visit to parliament by the Canadian prime minister, Stephen Harper, have been arrested, say police.

Environmental campaigners against tar sands, a form of fuel that produces more emissions than conventional oil, have posted a video showing them on the roof of parliament, with a statement saying they wanted to disrupt the premier's address to MPs and peers.

The Love Canada Hate Tar Sands site said: "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 spilled molasses on the floor outside of Parliament."

Shortly before Harper's speech the doors slammed shut on the robing room, where the Queen puts on the imperial state crown and her ceremonial robes for the state opening of parliament, and shouting was heard.

The activists said in a statement: "Cameron's government opens its arms to Harper and his cronies … Harper should be shamed internationally but he is instead invited to address both houses of parliament. Harper has taken Canada down a dangerous climate path, destroyed whole ecosystems and overriding centuries-old treaty rights."

In a separate protest, the UK Tar Sands Network held a demonstration outside parliament as Harper arrived, with around 50 people campaigning against extraction of oil from the tar sands in the boreal forests of Canada.

The group wants the UK government to support EU measures to label tar sands as more polluting than conventional oil and to discourage its future import.