British police on Thursday arrested three environmental activists after they staged a protest against the historic visit of Canadian Prime Minister Stephen Harper to the Houses of Parliament.

The demonstrators claimed to have made it to the roof of parliament during a protest against Harper's speech to a joint meeting of the House of Commons and Lords, the first time a Canadian prime minister has been afforded such an honour since William Lyon Mackenzie King in 1944.

"They were arrested inside parliament premises and we are investigating reports that they reached the roof", said a police spokesman.

Environmentalists are opposed to Canada's energy policy, in particular the development of its tar sands.

Harper had lunch with British counterpart David Cameron, during which the pair "agreed on the need to redouble efforts to conclude the Canadian-EU trade agreement," according to Cameron's Downing Street office.

Cameron on Wednesday insisted that a tie-up was "close" and the deal formed a major part of Harper's speech.

"It remains our hope that we will soon achieve a Comprehensive Economic and Trade Agreement with the European Union," said Harper.

"For Canada, and for Great Britain as a member of the EU, this will be a historic step -- a monumental one, in fact."

He pointed to a joint Canada-EU study which concluded that the deal would increase two-way trade by 20 percent.

The leaders also discussed the "need to force political transition in Syria" in order to end the country's civil war, and set out key priorities for next week's G8 summit in Northern Ireland, said Downing Street.

Protest group "Love Hate Canada tar sands" wrote on their website that they had tried to interrupt Harper's message due to his "dangerous" policies.

"We have managed to climb onto the roof with T-shirts saying 'oil out of politics', 'stop Harper' and 'stop the tar sands'," explained the message.

The group also claimed that "two campaigners spilled molasses on the floor outside of parliament".

"Harper should be shamed internationally but he is instead invited to address both houses of parliament," said the group.

"Harper has taken Canada down a dangerous climate path, destroyed whole ecosystems and overriding centuries-old treaty rights," it added.

The Canadian minister carried on with his speech as normal, and expressed his gratitude at the invitation.

"For anyone who fully understands and truly cherishes the free and democratic nature of our institutions, and the long history upon which they rest, there is no honour to compare with an invitation to stand here, at the very cradle of our political system," he said.

He called on Syrian President Bashar Al-Assad to step down but warned that "the extremist, sectarian nature of much of the opposition cannot be ignored or wished away."

Harper also blamed "moral relativism" in world affairs for providing a shelter to anti-Israel sentiment, singling out "malevolent" Iran for particular criticism.