The alleged victim, 42, told cops two men held her down and assaulted her at 2am, during a party to mark squatters' last night at Alan and Ann Keen's derelict house.
The group had been at the property in Brentford, West London, for a fortnight, protesting about the Labour couple's expenses claims.
One reveller, who asked not to be named, said: "Everyone's shaken up - it's a horrible end to what was an important protest."
The Met Police confirmed it was investigating a serious sexual assault claim. A spokesman said: "All the occupants of the house have been fully co-operative with police.
"There have been no arrests but we are taking statements from people who attended the party."
It is understood the woman told cops she was dragged upstairs before being attacked.
Officers had gone to the house earlier that night after neighbours complained about loud music being played in the street. One neighbour said: "The police persuaded them to take the music back indoors then left. The noise was really awful."
On Friday the Keens got a court order allowing bailiffs to evict the squatters from the £385,000 house within 24 hours.
Last week the group unfurled banners from the windows with messages such as "Fight political corruption". The Keens live in a luxury flat in Westminster but say it is their "second" home and claim up to £40,000 a year for it.
Mrs Keen says her "main" home is the Brentford property, which has been empty for almost a year. It has missing floors, half-built walls, no running water or electricity.
She previously blamed a dispute with builders, and told neighbours she and her husband were planning on moving in next month.
Mr Keen said last week the squatters should "get a job".
The couple are being probed by the Commons' sleaze watchdog.
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