PHILADELPHIA — Dozens of environmentalists blocked entrances to a federal office building Monday in Philadelphia to protest the Keystone XL oil pipeline and police arrested at least 24 people, organizers said.

About 40 activists stood in front of three sets of doors to the downtown William J. Green Building as another 100 people sang, chanted and waved signs in support of the civil disobedience. A fourth entrance remained open.

The actions were “a preview of the resistance to come” if President Barack Obama’s administration approves the $5.3 billion project, said Alexa Ross, spokeswoman for Earth Quaker Action Team, which organized the demonstration.

More than two dozen people were arrested by federal officers on charges of blocking a federal building and failure to obey an officer, Ross said. A spokesman for the arresting agency could not be reached Monday evening for comment.

The proposed 1,179-mile pipeline from Canada to Texas would create jobs and reduce the need to import oil from politically volatile countries, say supporters, who include lawmakers from both major political parties and many business and labor groups.

But protesters including Eileen Flanagan contend the project poses serious environmental hazards, such as carrying “dirty oil” that contributes to global warming.

“What climate scientists say about the world they’re going to inherit is really scary,” Flanagan said. “I feel like I need to stand up and do something.”

Obama has said he expects to make a decision on Keystone XL in the coming weeks. Secretary of State John Kerry must first make a recommendation on whether the project serves the national interest; Department of State approval is needed because the pipeline crosses a U.S. border.