Then, she wrote, her girlfriend was “in the middle of the bus fighting with them,” her face bloodied. Ms. Geymonat told the BBC that she had tried to intervene, and was quickly assaulted. “The next thing I know is I’m being punched,” she wrote on Facebook. “I got dizzy at the sight of my blood and fell back.”

Ms. Geymonat said that she may have lost consciousness, and that at some point the bus suddenly stopped. “The police were there and I was bleeding all over.”

A photo posted to her Facebook page shows Ms. Geymonat, her white shirt spattered red and a gash on her nose, and her girlfriend, her own face red.

Ms. Geymonat did not immediately respond to a request for comment on the attack or the arrests.

“This was a disgusting attack on two women who appear to have been picked out and targeted by a group of youths,” Detective Superintendent Andy Cox said in a statement. “The suspects have made a number of homophobic comments towards the couple before throwing coins at them. When the women tried to reason with the group, the attack escalated to an assault.”

He added that detectives were reviewing CCTV footage of the attack, and that “a number of active inquires are in hand to trace other individuals suspected to have been involved in the incident.”

Mayor Sadiq Khan of London condemned the attack, writing on Twitter: “This was a disgusting, misogynistic attack. Hate crimes against the L.G.B.T.+ community will not be tolerated.”

Jeremy Corbyn, the leader of the opposition Labour Party, called the attack “absolutely shocking.”

“We must not, and will not, accept this homophobic and misogynist violence in our society,” he tweeted.

In the radio interview, Ms. Geymonat said that she had never been physically attacked before.

“I have gay friends who have been in the streets and they have been punched,” she said. “I know that there is a lot of violence, verbal violence, like all the time, and that’s the thing that, like, made me tell the story, you know? Even when these guys came, it was not the first situation when men see two women kissing and they start acting as if we were a joke.”