The mayor forced organizers of the city's St. Patrick's Day parade to invite an LGBT veterans group to march two years ago, the parade group's lawyer said.
The lawyer, Chester Darling, made the accusations in an amended complaint filed this week to an earlier civil lawsuit accusing Mayor Martin Walsh of strong-arming organizers in 2014 with thinly veiled threats to withhold permits.
The parade has long been embroiled in legal controversy, including a 1995 U.S. Supreme Court ruling allowing members of the South Boston Allied War Veterans Council to exclude gay groups on free speech grounds.
The current lawsuit claims that Walsh ignored the court's order and violated the council's civil rights.
Walsh maintains that he was well within his legal rights to encourage organizers to allow OutVets to march.
"It's not true. It's completely not true," Walsh told the Boston Herald in response to the new complaint. "I don't know what veterans are claiming that, but they should call me if there's an issue."
According to the complaint, the mayor at one point got into a profanity-laden shouting match with parade organizer Philip Wuschke, and later apologized in a voicemail that Darling provided to The Boston Globe.
"Hands off our parade, that's all we want," Darling told the newspaper.
Walsh ended his personal boycott of the parade last year when the organizers agreed to let OutVets participate.
In March, a federal judge ruled against the city's bid to shorten this year's parade route. Police Commissioner William Evans cited public safety concerns in seeking the shorter route, but the complaint alleges the move was retaliatory on Walsh's behalf.
This is not the first time the Walsh administration has been accused of using heavy-handed tactics with organizers of an event in the city. Two City Hall officials were indicted earlier this year on extortion charges for allegedly pressuring a music festival to hire union stagehands.