The opposition Conservative Party expressed outrage Friday over the arrest of a key lawmaker as part an investigation into the leak of secret government information.

Damian Green, who is the party's spokesman on immigration issues, was taken into custody Thursday and released nine hours later in an operation that involved anti-terrorist police. Conservatives said the arrest was related to the publication of stories about Britain's Home Office, which is responsible for managing Britain's borders and immigration.

London Mayor Boris Johnson, a Conservative, said it was "hard to believe that on the day when terrorists have gone on the rampage in India that anti-terror police in Britain have apparently targeted an elected representative of Parliament for no greater crime than allegedly receiving leaked documents."

The case is unusual because opposition politicians frequently exploit information leaked by civil servants without becoming subjects of police investigations.

The Home Office has suffered a string of embarrassing leaks over the past year — including the revelation that an illegal immigrant had been employed as a cleaner in Parliament, and that 5,000 illegal immigrants were working as security guards and bouncers in the U.K.

A 26-year-old Home Office official was arrested on Nov. 19 on suspicion of misconduct in public office in connection with the same investigation into the leaks, police said in a statement. The official, whose name was not disclosed, was released pending further questioning in January.

Asked about Green, London's police force issued a statement saying a 52-year-old man was arrested on suspicion of "conspiring to commit misconduct in a public office." Police said he was released on bail without being charged, pending further questioning in February.

Metropolitan Police said they were investigating an "alleged leak of confidential government material" and their decision to make the arrest was taken "without any ministerial knowledge or approval."

Police said there was no suspicion of any terrorist offense, but that the investigation fell within the miscellaneous responsibilities of the Counterrorism Command.

British Prime Minister Gordon Brown's office said he had no prior knowledge of the move. Brown's Labour Party described the arrest as a police matter.

Conservative Party leader David Cameron described the police operation as "heavy-handed."

Green, a former financial journalist, was elected to Parliament in 1997 and re-elected in 2001 and 2005. He was recruited to serve as the Conservatives' spokesman on immigration issues in 2005.

He argued that opposition politicians have a duty to hold the government accountable.

"I have many times made public information that the government wanted to keep secret, information that the public has a right to know," he said.

British law protects whistle-blowers under certain conditions, such as in cases of reporting crimes or threats to public health and safety.

Four years ago, Katharine Gun, a translator at the secret Government Communications Headquarters, leaked a confidential memo from U.S. intelligence officers asking their British counterparts to spy on members of the U.N. Security Council before the Iraq war. The prosecution dropped the case when it came to trial.