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 – which was filmed by TV news cameras – happened after climate change protesters disrupted the beginning of Chancellor Philip Hammond’s annual Mansion House speech to the City 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.
Video footage shows Mr Field getting out of his seat and stopping one female protester by pushing her against a column and then marching her out of the room.
City of London Police said: “We have had a number of third-party reports of a possible assault. These are being looked into.”
Conservative Party chairman Brandon Lewis told ITV’s Good Morning Britain that the party would investigate the “full details of what happened”.
Greenpeace climate campaigner Areeba Hamid told the BBC the activist was “in shock” last 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.
“We went into that event announcing who we were very clearly, we were there to make a very important point – that we are in a climate emergency – to a roomful of people who are actually capable of making some very profound decisions.”
She said the activist is trained and experienced in non-violent direct action. Greenpeace activists were prepared to face legal consequences for their actions, Ms Hamid said, but “what they have not signed up for is physical assault”.
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 was among those who criticised Mr Field, tweeting: “This is horrific… He 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.”
And Jess Phillips MP tweeted: “Every MP has to deal with protest and conflict, it is done with words. To watch this is so so awful.”
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 one of Mr Field’s Conservative colleagues, Sir Peter Bottomley MP, said he had done nothing wrong.
“I think Mark Field did the right thing. He prevented the woman going further, he reversed her direction and she went out pretty willingly,” he told the BBC.
“He intervened – I congratulate him for that, I would have done the same. Although she may have been harmless, others won’t be.”
Sir Peter said that 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,” he said.
‘Guests felt threatened’
In a statement given 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.
“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.”
He added he would refer himself to the Cabinet Office and pledged to fully co-operate with its investigation into whether he had broken the ministerial code.
Watching the clip for the first time on BBC Newsnight, former Foreign Office minister Alistair Burt said: “I’ve no appreciation of the context of anything like that. Mark will answer for himself. But it looks to be a very difficult situation for everyone concerned.”
Meanwhile, Conservative MP George Freeman tweeted: “This looks appallingly rough handling of a woman climate protester in a dress.
“But before everyone rushes to instant armchair judgement can I suggest that all of us who weren’t there and don’t know what was said or done just wait a few hours to hear what those who *were* there say.”
Tory MP Vicky Ford defended Mr Field’s actions, saying the protester was “rushing” towards the governor of the Bank of England. “If she’d been carrying a weapon we’d all be hailing him a hero,” she said.
The protesters were removed from the event after several minutes and Mr Hammond was able to continue soon afterwards.
City of London Police said they received a call from security at Mansion House just after 21:00 BST.
“Officers arrived to help with their ejection. Once in the presence of the police, the protesters were co-operative and left the premises. No arrests were made.”
Resuming his speech, Mr Hammond told the guests: “The irony is that this is government that has just led the world by committing to a zero carbon economy by 2050.”