The incident took place in the early hours of May 30 after the women, in their 20s, boarded a night bus in West Hampstead.
“As they sat on the top deck, they were approached by a group of four males who began to make lewd and homophobic comments to them,” a police statement said.
“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.”
Both women were treated in hospital for facial injuries.
Four men aged 15 to 18 have been arrested, police said later, and they were looking for other suspects.
Melania Geymonat, 28, told BBC radio the men began harassing them when they discovered she and her girlfriend Chris were a couple.
“They surrounded us and started saying really aggressive stuff, things about sexual positions, lesbians and claiming we could kiss so they could watch us,” she said.
“To ease the situation I tried to make some jokes like Chris wasn’t understanding because she didn’t speak English. She even acted as if she was sick... but they started throwing coins. The next thing I know Chris is in the middle of the bus and they are punching her.
“So I immediately went there by impulse and tried to pull her out of there and they started punching me. I was really bleeding.”
Detective Superintendent Andy Cox said: “This was a disgusting attack on two women who appear to have been picked out and targeted by a group of youths.
“The suspects have made a number of homophobic comments toward the couple before throwing coins at them,” he added in a statement. “When the women tried to reason with the group, the attack escalated to an assault.”