MP Eric Joyce has been suspended by the Labour Party after being arrested on suspicion of assault after an alleged fight in a bar in the House of Commons.
A spokesman for the party said the suspension would be maintained until a police investigation into the incident was completed.
Sources said the decision was taken by Labour leader Ed Miliband.
Bar staff, who were on duty on Wednesday evening, were interviewed at Belgravia Police Station Police on Wednesday afternoon, and police began interviewing MPs, who were present during the incident, in the House of Commons in the evening.
Police are understood to have been called to Strangers Bar in the Commons at about 11pm on Wednesday.
Stuart Andrew, Conservative MP for Pudsey , has alleged he was assaulted and headbutted by Mr Joyce, MP for Falkirk.
He told Sky News he received a "bloody nose" and intends to press charges.
MPs say several people tried to intervene in the alleged incident, including Labour whip Phil Wilson, who witnesses told Sky News was seen with blood on his shirt.
A Labour source alleged Mr Joyce went "beserk".
It is understood Mr Joyce was handcuffed in the bar and taken to a "cell" in the House of Commons.
The glass in a door to this room is currently broken.
Scotland Yard confirmed officers detained a man in his 50s. He is currently being held in custody at a central London police station.
Commons Speaker John Bercow said the incident would be taken seriously but called on MPs not to discuss it in the Chamber.
"Members will be aware of reports of a serious incident in the House last night," he said.
"I have been informed by the Serjeant at Arms that the honourable member for Falkirk has been detained in police custody.
"The matter is being investigated. I take this matter very seriously, as do the House authorities," he added.
Mr Joyce, the MP for Falkirk since December 2000, served in the Army Education Corps before pursuing a career in politics.
In 2010, he resigned as shadow Northern Ireland minister after pleading guilty to failing to provide a breath test.
From 2003, Mr Joyce served as a Parliamentary Private Secretary (PPS) to a number of government ministers.
Mr Joyce quit as the PPS to Bob Ainsworth in 2009 due to his concerns over the war in Afghanistan.
Prior to that he had been a parliamentary aide to John Hutton, including the period when he was defence secretary, Mike O'Brien and Margaret Hodge.
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