A pervert who exposed himself and performed a sexual act in front of a teenage girl was promptly arrested by an off-duty police officer.

The Detective Inspector - based in Hackney and Tower Hamlets - was travelling by train to work a night shift on November 29 when he was approached by a distressed teenager.

The girl, who sought his help despite not knowing his occupation, told him a man on the train had indecently exposed himself and begun committing a sexual act.

The DI asked the victim to point out the suspect, who was on another carriage, and immediately arrested him as the train pulled into New Cross station.

As they got off the train, the arrested man tried to break free and became violent towards the officer, leading to a tussle on the platform.

A member of the public and station staff came to help the officer detain the man and called 999 for assistance.

British Transport Police (BTP) officers attended the scene and took the man into custody.

The officer spoke to the girl’s father on the phone and waited with her until he arrived. She is being supported by specialist officers.

The arrested man has since been convicted of the offence following an investigation led by BTP.

Detective Chief Superintendent Marcus Barnett, who is in charge of policing in Hackney and Tower Hamlets, said: “Being a police officer means we are expected to step up and place ourselves in difficult or dangerous situations in order to protect the public, regardless of whether or not we are on duty.

“My officer acted extremely fast and prevented the suspect from getting away while still supporting the victim who was understandably extremely shaken.

“I hope this shows the public our commitment to keeping everyone safe, especially women and girls.

“I am incredibly proud of the selfless and brave way in which he stepped forward, not thinking for a moment about anything other than protecting the young girl and the wider community.

“I also applaud the actions of the member of the public and station staff who stepped in and supported the officer when he shouted for assistance."