Police have used CS spray and arrested a man after a fight between guests at a reception in the Houses of Parliament.
The scuffle involved two guests at a spring drinks reception for the media in the shadow cabinet room, hosted by Conservative chairman Eric Pickles.
The guests left the event but continued arguing and were stopped in the corridor by a police officer.
The man was arrested and taken to a nearby police station for questioning and later released on bail.
BBC Political correspondent Iain Watson was at the drinks event
Conservative Party chairman Eric Pickles had light-heartedly lectured his 40 or so guests crammed into the shadow cabinet room in the Commons on the dangers of binge drinking.
The guests came from across the political spectrum - a Morning Star journalist here, a socially conservative broadsheet columnist there.
It seemed a well behaved, enjoyable if fairly low key, event. Most of us were taking the opportunity to talk to the Conservative chairman about the big issue of the day - MPs' expenses.
The shadow cabinet room is at the very periphery of the Palace of Westminster and is linked, though a series of doors, to the rest of the complex by a long corridor which emerges close to the back of the Commons chamber, near the Speaker's chair. It was here that the incident happened.
But none of us at the event - wary of the binge-drinking warning of course - had any indication of something being wrong. The first I heard was when a BBC colleague was prevented by police from leaving the Commons because "CS gas had been let off".
Some of the guests at the event told the BBC they thought it was an over-reaction but Commons sources said a police officer received minor injuries in the incident.
A Metropolitan Police spokesman said: "We understand two 'non-passholder' guests began arguing at a reception. Both men left the reception and subsequently entered a corridor in a private area of the House where they were stopped by a male police officer.
"One of the men became aggressive and allegedly punched the officer in the face - he sustained a split/cut lip.
"CS Spray was used by the officer to restrain the 40-year-old man who was arrested on suspicion of assault."
The man, who has not been named, was released on bail until 7 July pending further inquiries.
Commons authorities confirmed the incident had not been terrorist related or linked to the G20 meeting of world leaders in London later this week.
It is understood that the incident happened in the corridor behind the Speaker's chair, at the same time as MPs were debating Africa.
Labour MP Kerry McCarthy wrote on Twitter that she had heard the scuffle from within the chamber, adding: "We thought it was a protest, not a punch-up."
Her message said: "Was in chamber... Much excitement, banging and shouting."
Sources said parliamentary sittings had not been disrupted.
The incident, involving two people who did not have House of Commons passes, will raise fresh questions about security at Westminster.
Parliamentary security has been breached on several occasions in recent years, notably when pro-hunting supporters invaded the Commons chamber in 2004.
Last year protesters from the Plane Stupid campaign group scaled the roof of Parliament to demonstrate against new airport runways.
Mr Pickles was appointed party chairman in January, having previously been shadow communities secretary. He was hosting a reception for journalists.
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