* Police arrest three protesters at parliament in London
* Demonstrators target visit by Canadian prime minister
* Organisers say tar sands activists scaled roof
By Peter Griffiths
LONDON, June 13 (Reuters) - British police arrested three environmentalists on Thursday after protest organisers said they scaled the roof of parliament during a visit by Canadian Prime Minister Stephen Harper, in an embarrassing security breach before Britain hosts a G8 summit.
Two men and a woman were held on suspicion of trespassing on a protected site minutes before Harper gave a speech to both Houses of Parliament as part of his visit to Britain, London's police force said in a statement.
Harper was in London before attending a G8 meeting in Northern Ireland with British Prime Minister David Cameron, U.S. President Barack Obama and other world leaders on June 17-18.
Organisers of a protest against Canada's extraction of crude from oil sands said members of their group had managed to scale the roof of the parliament building in Westminster. Police said they were aware of reports the group had climbed on to the roof.
Two more protesters were arrested outside one of the main entrances to parliament on suspicion of criminal damage. Protesters said they had thrown molasses, a dark heavy syrup that looks like oil, onto the floor.
The Gothic parliament building is one of the capital's most heavily policed locations, surrounded by metal car barriers, tall fences and security gates. Armed police routinely patrol the area.
Parliamentary authorities said the protest had not disrupted proceedings inside. Security appeared to have been tightened after the incident, with armed police standing guard at the entrances.
"Three protesters were detained at 12 noon by parliamentary security staff while trying to access non-public rooms in parliament," a parliamentary spokesman said. "The business of parliament was unaffected. A full investigation is underway."
Organisers posted a statement online saying protesters evaded security to reach the roof of parliament. They wore T-shirts saying "Oil out of politics", "Stop Harper" and "Stop the tar sands", the statement said.
"Harper has taken Canada down a dangerous climate path, destroying whole ecosystems and overriding centuries-old treaty rights. Now he is trying to stain Europe too," it added.
Protesters outside parliament waved banners saying "No dirty oil" and "Harper - Planet Destroyer".
Extracting crude from the clay-like oil sands requires more energy than conventional oil production. Environmentalists say that increases greenhouse gas emissions, making the oil sands a top target for the green movement.
Activists protested against climate change on the roof of parliament in 2009 and against the expansion of London's Heathrow Airport a year earlier. (Editing by Mike Collett-White)