Greenpeace has accused Foreign Office minister Mark Field of assault, after he pushed a female activist out of a black-tie City event.
Mr Field claimed he reacted “instinctively” and has referred himself to the Cabinet Office for an investigation.
He also apologised to the woman for “grabbing her” but said he was worried she may have been armed.
The Tory MP has been widely criticised but some people defended his actions.
Thursday night’s incident – filmed by TV news cameras – happened after climate change protesters disrupted the beginning of Chancellor Philip Hammond’s annual Mansion House speech about the state of the UK economy.
Dozens of activists – dressed in suits, red dresses and sashes – “gatecrashed” the dinner, according to Greenpeace, and refused to leave.
Footage shows Mr Field getting out of his seat and stopping one female protester by pushing her against a column and marching her out of the room.
City of London Police said it was looking into “a number of third-party reports of a possible assault”.
And Conservative chairman Brandon Lewis told ITV’s Good Morning Britain the party would investigate the “full details of what happened”.
Greenpeace campaigner Areeba Hamid told the BBC the activist had been “in shock” on Thursday night, but was recovering and had been reassured by the “outpouring of support” online.
“I think Mark Field should have a long hard stare at himself and think whether that behaviour is in keeping with someone in public office,” she said.
It was “quite ludicrous” to suggest that the protester might have been armed, Ms Hamid added.
Labour’s shadow women and equalities minister Dawn Butler tweeted: “This is horrific… [Mark Field] must immediately be suspended or sacked.”
Fellow Labour MP Tonia Antoniazzi added: “No one who reacts like this to a peaceful protest should be sitting in our Parliament.”
Meanwhile, Liberal Democrat MP Chuka Umunna described Mr Field’s actions as “totally unacceptable” while former Tory MP Sarah Wollaston said it was “absolutely shameful, a male MP marching a woman out of a room by her neck”.
But Conservative MP Johnny Mercer defended Mr Field, tweeting: “Honestly? Try being in our shoes in the current environment.
“He panicked, he’s not trained in restraint and arrest, and if you think this is ‘serious violence’ you may need to recalibrate your sensitivities. Calm down, move on, and be thankful this wasn’t worse.”
Fellow Tory MP Sir Peter Bottomley said attacks on MPs and their staff meant that such protests could not be ignored. “Not intervening often has a cost, and if this becomes a fashion, there will be casualties.”
In a statement to ITV News, Mr Field – who is the Foreign Office minister for Asia and the Pacific – said: “In the confusion many guests understandably felt threatened and when one protester rushed past me towards the top table I instinctively reacted.
“There was no security present and I was for a split second genuinely worried she might have been armed.
“As a result I grasped the intruder firmly in order to remove her from the room as swiftly as possible.”
He added: “I deeply regret this episode and unreservedly apologise to the lady concerned for grabbing her but in the current climate I felt the need to act decisively to close down the threat to the safety of those present.”
Mr Field added he would refer himself to the Cabinet Office and fully co-operate with its investigation into whether he had broken the ministerial code.