Protesters built a giant pipeline to greet Justin Trudeau in London

The stunt, which blocked the main entrance to the Canadian High Commission, was orchestrated by Greenpeace activists who oppose a multibillion-dollar pipeline planned in western Canada across indigenous lands.

The Trans Mountain expansion, approved in 2016, will help oil companies reach new markets by expanding the capacity of North America's only pipeline with access to the West Coast, and it will nearly triple the number of oil tankers traveling the shared waters between Canada and Washington state.

Greenpeace activists said the project will take highly polluting tar sands to global markets, and that building the pipeline would make Trudeau's claims of climate leadership a "laughingstock."

"Trudeau is risking the health of Canada's rivers and coast, the water supply and livelihoods of many indigenous people, and undermining the credibility of the Paris climate agreement, all to keep some struggling oil companies in profit," Greenpeace UK oil campaigner Sara Ayech said in a news release.

According to the project's overview, the expansion will create a system similar to an existing 714-mile pipeline in Canada between Strathcona County in Alberta and Burnaby in British Columbia. That will turn the the capacity of 300,000 barrels of oil per day to 890,000 barrels per day, the overview says.

The project is estimated to cost $7.4 billion (Canadian) and government officials say it will be beneficial, creating jobs and tax revenue.

But it is opposed by the indigenous people of the First Nation communities in Canada, and there have been protests in several Canadian cities. The leader of the province of British Columbia and the governor of Washington state also have opposed the pipeline.

Greenpeace said 30 volunteers assembled the mock pipeline in about 15 minutes and then took about an hour to decorate the columns of the Canada House building, home of the Canadian High Commission, with banners that read "Crudeau Oil."

Thompson said the protest pipeline was installed around 6 a.m. and went from the main entrance of Canada House to the consular entrance on Cockspur Street, and Trudeau arrived around 8:30 a.m.

"The pipeline is still there, but the High Commission removed the banners. Our activists left after 12 hours of protests. The police were very polite and helpful," Graham Thompson, a Greenpeace spokesman, said later Wednesday.

Trudeau was in London for the Commonwealth Heads of Government Meeting.

On Sunday, he told the CBC the Trans Mountain expansion will be built despite the protests and the British Columbia government's continued battle against it in the courts. He says the project is in the country's best interests.