Librarians say a group against mask mandates has been harassing and intimidating workers and patrons at Boston Public Library branches.
Library staff say the group, which has posted videos of the incidents on social media, has refused to leave when asked.
Masks are required in indoor public spaces in Boston to help slow the spread of COVID-19.
Workers have had to call police.
"To try and fight back at policies that have been in place for so long, at this point, feels like a useless, uphill battle that they're trying to fight," said library patron Stells Del Tergo. "I don't think public libraries are the place for that."
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NBC10 Boston tried to reach the group Friday, but one person hung up the phone, and there's been no other response so far.
There have been three incidents in the last few weeks — two at the Copley Square branch and one in Hyde Park.
"Because of the fact that they're filming, I would guess their intentions is to get exposure for their point of view," said Maty Cropley, president of the Boston Public Library Professional Staff Association.
In response to the tense encounters, a rally to show unity and kindness has been organized by library staff on Saturday at noon outside the Copley Square branch.