Jerome Bowes was involved in an altercation at Elephant and Castle
Staff at three London Underground (LU) stations are staging a 24 hour walk out over the sacking of a colleague.
About 100 members of the Rail Maritime and Transport (RMT) union at Elephant and Castle, Charing Cross and Lambeth North went on strike from 1900 BST.
The union claims Jerome Bowes was sacked for defending himself against an assault from a member of the public.
An LU spokesman said: "The decision to dismiss Mr Bowes was upheld on appeal and reviewed at director level."
He added: "We do not tolerate any assault, verbal or physical, on staff or passengers.
"London Underground is disappointed that the RMT has voted in favour of striking because it serves no purpose.
"But the industrial action is limited and LU have put in places all reasonable measures to ensure that passengers are not affected."
'Angry and disgusted'
RMT general secretary Bob Crow said members at the three Bakerloo Line stations, who voted by a margin of nine to one to strike to demand Mr Bowes' reinstatement, will not book on for shifts.
"Jerome Bowes was sacked for defending himself against an attack from behind from someone who had already been involved in a fight on the most volatile night of the year, and that is unacceptable," Mr Crow said.
"Neither the Metropolitan Police nor the BTP found any cause to arrest Jerome, who was walking away towards a place of safety when he was assaulted, and it beggars belief that LU has seen fit to dismiss him.
"The message LU seems to be sending out is that defending yourself against violent assault is to be regarded as a disciplinary offence, and it is no wonder that our members are angry and disgusted."