MP Eric Joyce spent the night in custody at a central London police station after an alleged drunken brawl at a House of Commons bar.
The independent MP for Falkirk is still being held by police after the reported fracas during a karaoke night at the Sports and Social Club.
Officers were called to a disturbance just before 22:30 GMT on Thursday.
Mr Joyce, 52, was convicted of assault in a parliamentary bar last year, and later resigned from the Labour Party.
The Sports and Social Club was said to be packed 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."
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."
Asked whether the prime minister felt Mr Joyce could continue as an MP, David Cameron's official spokesman told a regular Westminster media briefing: "That is a matter for Mr Joyce."
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, he 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, Mr Joyce headbutted Tory MP Stuart Andrew and attacked three other politicians.
While offering an apology in the Commons over the incident, he said: "Clearly I have a number of personal issues to address, and you can be assured that this will take place."
Speaking outside court, after pleading guilty to assault in March last year, Mr Joyce said the fight was a "matter of considerable personal shame" and that the punishment was fair.
"Drink was an aggravating factor, there's no question about that," he said.
"It's something I have to deal with personally. Not everyone who drinks gets involved in fights and certainly when they are my age."