British Transport Police officers are appealing for information after an off-duty special constable was assaulted at London Bridge Station, leaving him with a fractured nose.
Investigators have released a CCTV image of a man they would like to speak to in connection with the incident, which took place on Friday 22 March.
"The victim witnessed a man pulling a woman through London Bridge Station around 8.20pm," said PC Jonathan Prettejohns.
"When the victim stepped in to intervene and explained he was an off-duty officer, he was violently headbutted by the man, causing a fractured nose.
"Leaving the victim suffering from his injuries, the man continued pulling the woman through the station before boarding a Southeastern train."
It is believed the man entered the station with the woman via Joiner Street. Officers are conducting a number of enquiries into the incident, including viewing CCTV in and around the area and speaking to witnesses.
PC Prettejohns said: "The victim was left with a fractured nose and severe swelling as a result of stepping in.
"This violent behaviour is simply unacceptable and I'd urge anyone who recognises the man pictured, or indeed if you are the man, to come forward and assist officers in the investigation."
Anyone with information is urged to contact British Transport Police (BTP) on 0800 40 50 40 quoting background reference B8/LSA of 04/04/2013. Information can also be passed to the independent charity Crimestoppers, anonymously, on 0800 555 111.