Greenpeace protesters have climbed a 270ft construction crane streets from the White House and unfurled a massive orange and yellow banner featuring the word "resist".
The protest came the morning after US President Donald Trump issued orders to delay environmental rules and restart pipeline projects.
A spokesman for the environmental group, Travis Nichols, said the protesters are encouraging resistance to Mr Trump and his agenda.
A Greenpeace statement said the demonstration is "calling for those who want to resist Trump's attacks on environmental, social, economic, and educational justice to contribute to a better America".
Police closed three city streets to traffic around the site on Wednesday morning, but officers on the scene appeared to be doing little more than monitoring the activists, who were secured with ropes and harnesses as they hung from the crane.
Captain Robert Glover, of the Metropolitan Police Department's special operations division, told reporters that seven people were on the structure.
"Time is on our side," Mr Glover said. "Safety is our foremost mission this morning."
John Evans, 46, a carpenter who works on the construction site - previously home to The Washington Post - said the protesters must have arrived before workers turned up at 5am.
"We didn't see them climb up," he said.
Mr Evans said the protesters were clearly experienced, noting that they were moving their legs and shifting positions to maintain their blood circulation.
"Look how organised they are. They have the same equipment that I use every day," he said.
"They're professionals. Amateurs couldn't stay up there that long."
The protest comes a day after Mr Trump signed orders intended to restart construction of two oil pipelines, the Dakota Access and the Keystone XL.
Former president Barack Obama halted the Keystone XL pipeline in 2015 and the Army Corps of Engineers blocked the construction of the Dakota Access pipeline in December after months of protests led by the Standing Rock Sioux tribe, which gets drinking water from a reservoir in the pipeline's path.
Also on Tuesday, Mr Trump's administration moved to delay implementation of at least 30 environmental rules and froze new Environmental Protection Agency contracts and grant awards.
Police said on Twitter that the protesters' actions were "extremely dangerous and unlawful".
Mr Glover said police would consult with prosecutors about appropriate charges.
A few dozen people were standing outside the site, taking photos, but many just paused briefly before moving on.
David Presgraves, 27, and Victoria Oms, 26, who work nearby in non-profit communications, said they agreed with the protesters' message.
They both participated in the Women's March in Washington on Saturday.
"The pipelines have got to stop," Mr Presgraves said.
"There's no respect for the native people, no respect for the environment."