A newspaper photographer filed a complaint with police on Tuesday accusing the "30 Rock" star of punching him outside a marriage license bureau.
No charges had been filed, and the actor vehemently denied throwing any punches.
Daily News photographer Marcus Santos said that Baldwin became enraged as he and other photographers were snapping pictures of the 54-year-old actor and his fiancee, Hilaria Thomas, outside the lower Manhattan office where marriage licenses are obtained.
Santos told the newspaper, "I knew he was going to attack me. I stepped back, and he kept coming." He claimed the actor shoved him before punching him "one time, right in the chin."
A statement on Tuesday from Baldwin's publicist accused the photographer of being the aggressor.
As Baldwin and his fiancee were leaving, a person "walking in front of Alec positioned himself to obstruct the view of a photographer aggressively trying to shoot the couple," the statement said. "The photographer was clearly frustrated, pushed past the bystander and assaulted Alec with his camera. There were no punches thrown, and any subsequent physical contact was simply Alec protecting himself."
Photos on the newspaper's website (http://nydn.us/M3XinW) appear to show Baldwin shoving the photographer.
A New York Police Department spokeswoman confirmed the allegation was under investigation.
Last year, an American Airlines flight was delayed by Baldwin's refusal to stop playing a cellphone game, and he was kicked off the plane. Baldwin later asked for forgiveness from fellow passengers but stopped short of apologizing to the airline or the flight attendant he later mocked on Twitter.
Baldwin posted Tuesday on Twitter: "A 'photographer' almost hit me in the face with his camera this morning." He also said paparazzi should be "waterboarded."
In 2008, Baldwin blamed a bitter custody battle with ex-wife Kim Basinger in part for the anger and frustration he was feeling when he berated their daughter in a phone message that was leaked to the media.
In the message, Baldwin called their daughter, Ireland, a "rude, thoughtless little pig." He apparently was upset that she had missed his phone call. He said he apologized to Ireland and said the message was wrong and "horrified" him.
Associated Press writer Colleen Long contributed to this report.