Fresh arrests Sunday marked a second day of protests at the White House by environmental activists who said they were staging a two-week sit-in in a bid to halt a proposed oil pipeline.

Demonstrators from the Tar Sands Action group and a coalition of local and national groups are hoping a campaign of civil disobedience will push President Barack Obama to deny a permit for a new pipeline stretching from Canada to the US Gulf Coast.

"Saturday's arrests and overnight jailings are already lighting a fire," Mike Tidwell, director of the Chesapeake Climate Action Network, said Sunday.

"More people are now inspired, determined, and committed to join," said Tidwell in a statement from jail, after he was arrested Saturday.

Tar Sands said on its website that 50 people were arrested Sunday at a sit-in outside the White House front fence on Lafayette Square when they declined a request by US Park Police to move. They joined the more than 50 people detained a day earlier.

The Park Police could not be reached for comment on the arrests.

The action is designed to pressure Obama to deny a permit for the $13 billion Keystone XL pipeline project due to stretch across 1,700 miles (2,700 kilometers).

The US government plans to decide by year's end whether to issue a permit for the proposed pipeline stretching from Canada to Texas.

The Keystone XL pipeline proposed by TransCanada would begin in Alberta in western Canada and pass through the US states of Montana, South Dakota, Nebraska, Kansas and Oklahoma before ending up in Texas at the Gulf of Mexico.

Tidwell said that "on Monday alone over 20 DC-area doctors, lawyers and students will be going to jail to chant, sing, and stop the pipeline. They'll be joining Nebraska ranchers and others nationwide. Word is spreading."

Many of the weekend protesters wore pro-Obama buttons, but said they would not support his reelection if his administration was not prepared to more urgently back a green agenda.

Obama is currently on a 10-day vacation at Martha's Vineyard, a posh resort island on the East Coast.