An off-duty police officer on a train through Lewisham helped a girl moments after a man performed a sexual act in front of her.

The Detective Inspector was travelling to work for a night shift on a train through Lewisham on Monday, November 29.

At around 6.30pm he was approached by a teenage girl in distress.

She told him that another man on the train indecently exposed himself and began committing a sexual act in front of her.

She asked him for help - unaware of the officer's occupation.

The officer asked the victim to point out the suspect, who was on another carriage.

The officer detained and arrested the man as the train pulled into New Cross.

As they got off the train the suspect tried to break free and became violent towards the officer, resulting in a tussle on the platform.

Moments later a member of the public and station staff came to help the DI detain the suspect and called 999 for emergency assistance.

Officers from British Transport Police (BTP) soon attended and took the man into custody.

The officer spoke to the girl’s father on the phone and waited with her until he arrived.

An investigation was led by BTP and the victim is being supported by specialist officers.

The suspect has since been convicted for the offence.

Detective Chief Superintendent Marcus Barnett, who is in charge of policing in Hackney and Tower Hamlets, where the DI is based 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.

"Due to the brave actions of everyone involved a dangerous individual was arrested.”

If you have been a victim of sexual assault report it to police on 101 or call 999 if the incident has just happened.

If the incident took place on a train or tube report to British Transport Police by texting 61016 or calling 0800 40 50 40.

Always dial 999 in an emergency.

Information can also be reported anonymously to Crimestoppers on 0800 555 111 or online at www.crimestoppers-uk.org.