Detractors say the project - which would carry oil from Canada to Texas - will increase the country's dependence on fossil fuels and contribute to climate change. Supporters contend it's a job creator that won't cause any significant environmental damage.
On Monday, about 40 protesters linked arms and sat down in front of three entrances to the federal building at Sixth and Arch Streets.
Thirty were eventually arrested by Federal Protective Service officers, some after they climbed over police barriers erected around one entrance. One protester, 80-year-old Rabbi Arthur Waskow, unable to climb over a barrier, slid under instead. He was not arrested.
Attendees sang hymns and wielded brooms (to "sweep away corruption"), cheering, "Another climate hero," as demonstrators were led away by police. Barring a few minor confrontations at the building's entrances, organizers said, they were pleased with the event.
The activists selected the site, they said, because the State Department has a domestic office inside, and the public comment session for the department's environmental impact report on the pipeline has just come to an end.
"KXL opponents turn from words to actions, saying no to the pipeline by putting their bodies on the line," protesters wrote in a statement.
A State Department representative said the office had no comment on the event.
"We want Obama to say no to the pipeline," said Jonathan Snipes, an organic farmer from Morrisville. "If it takes getting arrested to send that message, we're willing to do that."