MP Eric Joyce has been arrested after reports of a disturbance in a bar at the Houses of Parliament.
The independent MP for Falkirk was detained after an incident at the Sports and Social Club bar.
The Metropolitan Police said officers had been called just before 22:30 GMT, and the arrested man “remains in custody and inquiries continue”.
Joyce, 52, was convicted of assault in a parliamentary bar last year, and later resigned from the Labour Party.
In a statement on Thursday night, a Met police spokesman said: “Police were called shortly before 10.30pm this evening to reports of a disturbance at a bar within the House of Commons.
“Officers attended and a man aged in his 50s was arrested in connection with this incident. He remains in custody and inquiries continue.”
The Sports and Social Club was said to be packed for its regular karaoke night at the time of the incident.
Tony Grew, from the PoliticsHome website, witnessed the alleged altercation, telling ITV News he saw the MP on the ground “wrestling with two police officers”.
“There were around at least 40 or 50 parliamentary staff, shocked, watching this melee as it occurred.
“Bar staff were trying to push people back, telling them to stand back and let the police do their work,” added Mr Grew.
“I can vividly remember a policeman’s hat rolling on the ground towards me as I was watching this scene unfold.”
Mr Grew suggested the altercation could have occurred because Mr Joyce became upset he was not allowed to take a glass outside into the smoking area.
MPs have expressed concern for Mr Joyce’s welfare on Twitter.
Conservative Penny Mordaunt said: “MPs done great stuff for mental health but must ensure that colleagues who need help get it, esp if they are now sans party.”
Former GP Sarah Wollaston, who has led Tory calls for a minimum alcohol unit price, said: “Eric Joyce is just one of hundreds of thousands who have lost control of their drinking. Westminster is blind to drinking problems.”
But Dennis Canavan, who was an MP and MSP for Falkirk, said that it was time for Mr Joyce to stand down.
He told BBC Radio Scotland’s Good Morning Scotland programme: “Well, this is sad news and this is sad for Eric Joyce, but it is even sadder for the people of Falkirk who for over a year now have no effective representation in Parliament and they definitely don’t deserve this.
“It is time to go for the sake of his constituents and for his own sake, he obviously has a problem and he needs help for that problem.”
Following the fracas in February last year, Mr Joyce, a former soldier, announced he would not seek re-election in 2015.
After admitting four counts of assault, Joyce was given a 12-month community order in March that included a Friday-to-Sunday curfew and a three-month pub ban. He was fined £3,000 and ordered to pay £1,400 in compensation to his victims.
During the brawl in the Strangers’ Bar, Joyce headbutted Tory MP Stuart Andrew and attacked three other politicians.
While offering an apology in the Commons over the incident, Joyce said: “Clearly I have a number of personal issues to address, and you can be assured that this will take place.”