"I had a date with Chris," Geymonat posted on her Facebook page. "We got on the Night Bus, climbed upstairs and took the front seats. We must have kissed or something because these guys came after us."
She said a group of four men, one with a Spanish accent, the others with British accents, were "behaving like hooligans" and demanding they kiss "so they could enjoy watching".
"In an attempt to calm things down, I started making jokes. I thought this might make them go away. Chris even pretended she was sick, but they kept on harassing us, throwing us coins and becoming more enthusiastic about it," Geymonat posted on her Facebook page.
According to her account, the situation then escalated, and the men started punching them. "Suddenly the bus had stopped, the police were there and I was bleeding all over," she wrote.
Police said the incident took place at 2.30am, and "the women were then attacked and punched several times before the males ran off the bus. A phone and bag were stolen during the assault". The two were treated at a London hospital for facial injuries and released.
London Mayor Sadiq Khan called the incident "a disgusting, misogynistic attack" and said hate crimes against gay people would not be tolerated.
The opposition Labour Party leader Jeremy Corbyn called it "absolutely shocking," and promised "solidarity to Melania and Chris and to all in the LGBT+ community for everything they endure for simply being who they are".
Prime Minister Theresa May also denounced the attack.
Geymonat posted photographs of herself and her partner with bloodied faces and clothes.
Siwan Hayward, director of compliance, policing and on-street services at Transport for London, told the BBC that the assault was "sickening" and "utterly unacceptable," adding that "homophobic behavior and abuse is a hate crime and won't be tolerated on our network".
As news of the attack began to spread, thousands took to social media to express solidarity with the couple. "Melania and Chris" trended in the United Kingdom on Friday and many also explained "this is why we still pride". London Pride, a month of LGBT+ events culminating in a massive parade on July 6, begins this weekend.