Guillaume Mélanie, president of Urgence Homophobie, said he was leaving a restaurant in the centre of the city when the attack happened.
He suffered a broken nose and went to hospital for treatment.
The story prompted an outpouring of support and condemnation of frequent such attacks in the French capital.
Last week, two young women sitting on a public bench were insulted with homophobic slurs before being attacked. Earlier in October, two men who had been kissing in the city were set upon, both receiving injuries which required hospital treatment.
And in late September, comedian Arnaud Gagnoud was attacked by six people for hugging another man while leaving a play.
Mayor of Paris Anne Hidalgo tweeted her support to Mr Mélanie, saying "this series of homophobic acts calls for a collective outburst".
"It is out of the question to resign ourselves to this violence," she tweeted to him. "We will do everything to ensure that everyone is free and sees their rights respected in Paris."
Mr Mélanie was attacked near the Etienne Marcel station in Paris' second district - somewhere he considered it a "very gay-friendly" neighbourhood, he told French broadcaster BFMTV.
His group, Urgence Homophobie, specialises in helping homosexuals who are persecuted in Chechnya - a major news story in 2017 and early this year. Mr Mélanie and some friends had been celebrating a residence permit secured for one of the refugees, he told French media.
Leaving the restaurant, a man - "who had to see we were gay" - pushed one of the refugees out of his way, and was admonished by one of the group to be more gentle.
It was then, Mr Mélanie said, that the man hurled a series of homophobic slurs before launching his attack.
Mr Mélanie chose to upload a photo of his bloodied face and broken nose to Twitter, with the caption: "Tonight it's my turn."
The outpouring of support came from many in the LGBT community and beyond.
The former deputy mayor of Paris, Bruno Julliard, tweeted that the number of attacks showed "homophobia is thriving in 2018", and called for action from police, the judiciary, and educators.
Ian Brossat, a Paris councillor for the Communist Party, sent "all my support and solidarity" to Mr Mélanie, and said: "This unleashing of homophobia is monstrous."
And television presenter Christophe Beaugrand tweeted that he was "devastated by the tragedy" of what he a called a "scandalous homophobic assault".
Mr Mélanie, meanwhile, told well-wishers he was doing well in the emergency room overnight.
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