"They started behaving like hooligans, demanding that we kissed so they could enjoy watching, calling us 'lesbians' and describing sexual positions," she wrote on Facebook.
"It was only them and us there."
The next thing I know is I'm being punched
"They kept on harassing us, throwing us coins."
"The next thing I know is that Chris is in the middle of the bus fighting with them. On an impulse, I went over there only to find her face bleeding and three of them beating her up."
"The next thing I know is I'm being punched."
Source: Facebook: Melania Geymonat
Both were taken to hospital for treatment to facial injuries.
Police said the incident happened at around 2.30am on May 30 after the two women, both in their 20s, got on the bus in north London.
"They were approached by a group of four males who began to make lewd and homophobic comments to them," London's Metropolitan Police said in a statement.
"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."
They said four males aged between 15 and 18 were arrested on Friday on suspicion of robbery and aggravated grievous bodily harm.
The teenagers have been taken to separate police stations for questioning.
A fifth teenager, a boy aged 16, was also arrested on Saturday morning on charges of suspicion of robbery and aggravated grievous bodily harm.
"This was a disgusting attack on two women who appear to have been picked out and targeted by a group of youths," said Detective Superintendent Andy Cox.
"CCTV footage from the attack is being reviewed by detectives."
"Active enquires are in hand to trace other individuals suspected to have been involved."
"Attacks of this nature on London buses are rare."
Public support on social media for the couple expressed deep concerns over the alleged attack.
Geymonat is from Uruguay while British newspapers reported that her girlfriend is from the US.
British Prime Minister Theresa May said the assault was "sickening".
"Nobody should ever have to hide who they are or who they love and we must work together to eradicate unacceptable violence towards the LGBT community," she said.