Conservative immigration spokesman Damian Green has been arrested and released on bail in connection with a series of leaks from the Home Office.
Police say Mr Green was held on suspicion of "conspiring to commit misconduct in a public office".
The MP denied any wrongdoing and said "opposition politicians have a duty to hold the government to account" and that he would "continue to do so".
He was questioned, but has not been charged and was bailed until February.
Mr Green's arrest is believed to be connected to the arrest of a man suspected of being a Home Office whistleblower.
Speaking outside the House of Commons, Mr Green said: "I was astonished to have spent more than nine hours today under arrest for doing my job.
'Right to know'
"I emphatically deny I have done anything wrong. I have many times made public information that the government wanted to keep secret - information that the public has a right to know.
"In a democracy, opposition politicians have a duty to hold the government to account.
"I was elected to the House of Commons precisely to do that and I certainly intend to continue doing so."
The BBC understands that a junior Home Office official was suspended from duty 10 days ago over a number of leaks and the matter was referred to police. He was arrested but not charged.
It follows a series of leaks, including:
The BBC understands Tory leader David Cameron is angry about what has happened and stands by Mr Green.
'Big questions'
Speaking on BBC One's Question Time, shadow chancellor George Osborne said: "I think it's absolutely extraordinary that the police have taken that decision.
"It has long been the case in our democracy that MPs have received information from civil servants - I think to hide information from the public is wrong.
"Disclosure of this information was manifestly in the public interest"
"It is early days, it's an extraordinary case. I think there are going to be some very, very big questions asked of the police."
A spokesman for the Conservative Party said Mr Green had "on a number of occasions, legitimately revealed information which the Home Office chose not to make public.
"Disclosure of this information was manifestly in the public interest.
"Mr Green denies any wrongdoing and stands by his actions."
Conservative sources said a police investigation into a high-ranking politician would have to have been cleared at "the very top" and have described the actions as "Stalinesque".
'No prior knowledge'
But a Downing Street spokesman said: "This is a matter for the police. The prime minister had no prior knowledge of the arrest of Mr Green and was only informed after the event."
Mayor of London Boris Johnson has said he was informed in advance of the Met's plan to arrest Mr Green, and expressed "trenchant" concerns about the move.
Mr Green, the MP for Ashford in Kent, who has been the Conservatives' immigration spokesman since December 2005, was arrested shortly before 1400 GMT at his constituency home.
The Tories say they understand counter-terrorism officers were involved in searches of his home and offices.
In a statement, the Met said the arrest was made without any ministerial influence.
They said: "The investigation into the alleged leak of confidential government material followed the receipt by the Metropolitan Police Service (MPS) of a complaint from the Cabinet Office.
"The decision to make today's arrest was taken solely by the MPS without any ministerial knowledge or approval."
It said search warrants had been carried out at a home in Kent, a home in west London, business premises in Kent and in central London
"The search at the residential address in west London has concluded, the other searches continue," it added.
A Home Office spokesman confirmed that a Home Office official had been arrested last week "in connection with an inquiry into alleged leaked documents".
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