The "Stop the Steal" group, which called for "boots on the ground to protect the integrity of the vote," was adding 1,000 new members every 10 seconds and had grown to 365,000 members in a day.
"The group was organized around the delegitimization of the election process, and we saw worrying calls for violence from some members," a Facebook spokeswoman said.
Group backers said they were organizing peaceful protests, had been working hard to police the comments and that Facebook had given no warning. Chris Barron, a spokesman for the group, said political opponents were also organizing protests but were not banned.
"If Facebook wants to become the arbiter of truth then they've got a lot of work to do," Barron said.
Facebook said the group's removal was in line with "exceptional measures" amid "heightened tension."
Measures Facebook introduced on Thursday include warning users results are not final as well as limiting how many people are shown live video about the election, or posts that the company's algorithms believe contain political misinformation.
For months, Trump and Republican allies have been laying the groundwork to cast doubt on the integrity of the vote in case the president lost his re-election bid.
As tallies increasingly improve the odds for Trump's Democratic challenger, Joe Biden, and as U.S. broadcasters and other major media outlets brush off Trump's claims of victory, the president and his supporters have taken to social media to try to turn the narrative around, floating conspiratorial theories using the hashtag #StopTheSteal.
But social media companies have been signaling less patience with disinformation and calls for violence. Eleven of the president's 32 tweets since Election Day on Tuesday have been placed behind a warning label saying they were disputed, prompting him to use email and other media to voice his claims, researchers said.
"Social media platforms can't allow themselves to be used to foment anti-democratic and potentially violent activity," said Paul Barrett, deputy director of New York University's Stern Center for Business and Human Rights.
BANNON SUSPENDED
On Thursday, Snap Inc's Snapchat removed a video from Trump's account in which Biden said he has an extensive "voter fraud organization." Biden's statement came during an interview in which he was discussing his team fighting voter suppression efforts, and Snap determined that Trump's use out of context violated its policy against undermining the integrity of civic processes.
Trump campaign social media manager Ryann McEnany decried Snap's action in a tweet, saying in capital letters: "Why won't they let the American people see this!?"
Twitter on Thursday suspended an account used by former Trump campaign manager Steve Bannon after he recorded a video in which he called for beheading FBI Director Christopher Wray as well as government infectious diseases expert Anthony Fauci for being disloyal to Trump.
A Twitter spokesman cited company policy against glorifying violence. Other services also removed the video on similar grounds.
But the takedowns and warnings are something Trump supporters appear prepared for. Before Facebook deleted "Stop the Steal," organizers directed members to an email sign-up page "in the event that social media censors this group."
The group's membership surged because seven prominent conservatives promoted it to their hundreds of thousands of followers, according to Renee DiResta, a researcher involved in the anti-misinformation Election Integrity Partnership.
While Facebook groups typically work as forums for shared interests, they can harbor hyper-partisan misinformation.
"Facebook has been enabling and amplifying the infrastructure that's now being used to attack our democratic process," said Arisha Hatch, executive director of the Color of Change political action committee, one of the country's largest online racial justice groups.
A review of a small number of comments posted to "Stop the Steal" before its deletion found no direct calls for violence, but its organizing premise - that Republican votes are being "nullified" by Democrats - has no basis in fact.
Facebook, which normally recommends groups to users that they may want to join, last week suspended these recommendations for political groups and new groups around the election.
The "Stop the Steal" group was run by the Trump action group Women for America First. The non-profit organized protests against COVID-19 restrictions and supported Trump during his impeachment hearing.
