The police in London are investigating a case in which a UK-based Ghanaian gender activist was sexually assaulted by a man on a public train.

Dilys Sillah in an interview with Myjoyonline.com said the man repeatedly rubbed himself against her but she did not suspect he had a sinister motive.

The incident happened last Monday when she was rushing home from work after she was told a close friend’s husband had passed away unexpectedly.

She said she was devastated by the news and “was not in the best frame of mind” when she boarded the train.

Ms Sillah explained that when the man first touched her, she did not realise it was intentional because the train was packed to capacity during rush hour and passengers were very close to one another.

“When you're in a crowded space you don’t think you're going to be sexually assaulted,” she said adding, "So when you're repeatedly touched or whatever, you don't immediately know what's going on."

But she said the man touched her again for the second and third time, which compelled her to give him a look to indicate he was invading her space.

The man apologised she said, adding it was after the passenger repeated the act of rubbing himself against her that she realised what was really happening.

“I was so angry and I told him he was only sorry he'd gotten caught! I went on to tell him I was the last person on the train he should've done that to and proceeded to take his photograph," she said.

The gender activist said she could not ignore the incident because too many women in the UK are victims of sexual assaults on public transport, especially during rush hour, where these "sexual predators" attack when the trains are packed.

The London police have taken Ms Sillah's statement and she is helping them to identify the suspect.

Ms Sillah told the reporter sexual assault on women will stop if victims refuse to keep silent about their experiences because anyone can be a victim of a sexual crime.

"Sexual predators can only exist when we're silent," she said.