Sen. Kirsten Gillibrand, Sen. Brian Schatz, and Rep. John Lewis talk on the Capitol steps on Monday night Screenshot/Cory Booker Facebook Live A group of Democratic lawmakers, led by New Jersey Sen. Cory Booker, sat on the Capitol steps to discuss the GOP healthcare bill Monday night.

They rallied activists, random people passing by, and others critical of the Republican effort to repeal and replace the Affordable Care Act, more commonly known as Obamacare, in a marathon session broadcast on Facebook Live.

The informal gathering began with just Booker and Georgia Rep. John Lewis chatting on Facebook Live around 7 p.m. ET, and eventually attracted hundreds of people in person and tens of thousands on social media over almost four hours.

"By sitting in or sitting down, you're really standing up," Lewis, a 30-year veteran of Congress and civil rights leader, told Booker. "I say to people all the time, when you see something that is not right, not fair, not just, you have a moral obligation to stand up, to do something, to say something and find a way to get in the way and make a little noise."

The two were soon joined by tourists and members of advocacy groups, including NARAL Pro-Choice America, Planned Parenthood, and MoveOn.org.

Democratic lawmakers stopped by, too. Sens. Chuck Schumer, Kirsten Gillibrand, Chris Murphy, and Jeff Merkley all took turns condemning the GOP bill, which they argued would strip millions of America's most vulnerable of their health insurance and devastate middle and working class families. Meanwhile, Sens. Patty Murray of Washington and Mazie Hirono of Hawaii led a talk-a-thon on the Senate floor.

The lawmakers also asked members of the crowd to talk about how the bill could affect them and their communities, and representatives from social justice organizations discussed the law's implications for poor and marginalized groups.

The conversation swung from sober to light-hearted and boisterous, with Booker introducing Schumer as "Papa Smurf" to laughter and applause, as the minority leader took a seat on the steps.

Booker livestreamed the event on his cell phone for three hours and 40 minutes — an activist even gave him a phone battery to use so he could keep streaming. Senators also encouraged the crowd to tweet and Snapchat from the event.

"Light up the switchboard!" Sen. Dick Durbin called out at one point. "Get on the phones … don't be afraid to tweet."

Washington director of progressive political action group MoveOn.org tweeted updates on the rally throughout the night in a thread that got a lot of engagement:

Watch the full (nearly 4 hour) Facebook Live below: