A man has been arrested on suspicion of molesting a woman at Serangoon MRT Station last Wednesday.

The 23-year-old suspect allegedly started targeting the 28-year-old woman on board a city-bound train on the North-East MRT line.

The victim, who identified herself as Ms Karmen Siew, told The New Paper that she had boarded the train at Punggol Station at around 11.20pm.

She said a man who was sitting next to her started touching her thigh. The paper identified the man as a student from the National University of Singapore.

"He brushed his finger back and forth on my thigh. I was shocked but wondered if it was accidental," she said. "I crossed my legs to create more of a gap between us, but then he leaned in and did it again."

She quickly got up and moved to a reserved seat away from the man.

He remained in his seat, but when she alighted at Serangoon Station about 10 minutes later, he also got off and followed her.

As she was nearing the top of the escalator, she felt the man touch her from behind.

Ms Siew said: "I felt his finger run between my legs... I yelled a profanity at him because I was so stunned by what was happening."

She hurried to the control station to inform the service staff.

"I pointed at the man and yelled that he was a molester, hoping someone would stop him," she said. "But people just stared and did nothing."

The man then tapped out of the station and got away, she added.

Ms Siew made a police report that night and posted about the incident on her Facebook page.

Her post has had more than 1,000 shares as of Monday night.

The police said in a media release on Monday: "Through follow-up investigations, officers from Ang Mo Kio Police Division established the identity of the man and arrested him along Hougang Avenue 5 on Sunday.

"The suspect had allegedly molested the woman on board the train. He is believed to have molested the victim a second time after she alighted the train."

Investigations are ongoing.

If convicted of outrage of modesty, he can be jailed for up to two years and/or fined and caned.

Ms Siew said she hopes justice will be done and that people will come forward to help when they witness such incidents.