A 51-year-old man, who allegedly molested a woman on a bus, was trapped after the driver locked the doors to stop him from leaving.

The man was arrested for outrage of modesty and investigations are ongoing, the police told The New Paper yesterday.

The alleged victim said in a Facebook post that the incident happened after she boarded the bus at around 9.30am on Sunday in Telok Blangah Road, Shin Min Daily News reported.

"I hit his shoulder, yelled at him and asked the other passengers to call the police. The bus captain also saw the entire episode," she said.

She added that passengers rallied to protect her and scolded the man. "Although we are strangers, they offered their help and I am so grateful."

She said the man knelt down and begged her for mercy after the driver prevented him from leaving by locking the doors.

In the post, which has since been removed, she urged molestation victims to speak up to prevent such cases from being overlooked.