Deputy House of Commons Speaker and Conservative MP Nigel Evans has been arrested on suspicion of rape, sources said.
The 55-year-old Ribble Valley MP is alleged to have raped one man and sexually assaulted another between July 2009 and March 2013. Both alleged victims are in their 20s.
It is understood that Prime Minister David Cameron has been made aware of Saturday's arrest.
In a statement, Lancashire Police said a man from Pendleton, Lancashire, was being questioned at a police station in the county.
Born and raised in Swansea, Mr Evans has been MP for the Lancashire constituency since 1992. A popular figure at Westminster, in June 2010 he was elected as one of the three Commons deputy speakers. Later that year, he came out as gay saying that he was "tired of living a lie".
Pendleton residents told the Sunday Mirror they were stunned to learn of Evans' arrest. One neighbour said: "It's shocking. I've known him for 12 years, ever since I moved here. He's always been such a nice neighbour. There have been lots of police cars coming and going, which is not something we ever see here. The thought of him being arrested for rape is truly shocking. He's always seemed so pleasant."
A Lancashire Police spokeswoman said: "A 55-year-old man from Pendleton in Lancashire has today, Saturday, May 4 2013, been arrested by Lancashire Constabulary on suspicion of rape and sexual assault. The man will be interviewed at a police station in Lancashire during the course of the day. The offences are alleged to have been committed in Pendleton between July 2009 and March 2013."
There was no answer at Mr Evans' mid-terrace cottage in Pendleton on Saturday night.
Brian Binley, Tory MP for Northampton South and a friend of Evans, said: "I was just deeply disturbed and shocked. I've known him ever since I've been in Parliament and I came in in 2005. I consider him to be a very good friend. I know him to be caring, compassionate and in no way would he inflict himself violently on any other person.
"He is a good enough friend to have come up last Friday to a fundraiser my partner Sally and I were holding for Macmillan nurses and so I do know him well. I just hope and pray that this thing is cleared up sooner rather than later."