John Evans, 46, a carpenter who works on the site, said the protesters were clearly experienced climbers, noting that they were moving their legs and shifting positions to maintain their blood circulation.
"Look how organized 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 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, Trump's administration moved to delay implementation of at least 30 environmental rules and froze new Environmental Protection Agency contracts and grant awards.
Topakian said the action was intended to protest "all the things he's promised to do to push our country backwards."
A few dozen people standing in the streets below took photos, but many just paused briefly before moving on.
David Presgraves, 27, and Victoria Oms, 26, who work nearby in nonprofit communications, said they agreed with the message. Both participated in the Women's March on Washington on Saturday.
"The pipelines have got to stop," Presgraves said. "There's no respect for the native people, no respect for the environment."
One of the activists posted video online from the top of the crane.
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