© Leon Neal/Agence France-Presse - Getty ImagesRebekah Brooks, the former Chief Executive of News International,on July 1, 2011, at Wimbledon
The timing, two days before a separate parliamentary inquiry into the crisis, drew a skeptical response from opposition lawmakers who said the arrest might inhibit Ms. Brooks' ability or readiness to testify before the panel while she is the subject of police inquiries.
David Wilson, a lawyer representing Ms. Brooks, said she "maintains her innocence, absolutely."
The development was the latest twist in a series of events that has transformed Mr. Murdoch from a virtually untouchable force in the British media landscape to a mogul fighting for the survival of his power and influence.
Earlier on Sunday, the Labour opposition leader, Ed Miliband, who has taken a lead in assailing Mr. Murdoch's operations, called for the breakup of News International, the British subsidiary of Mr. Murdoch's News Corporation. Mr. Miliband called the newspaper group's influence "dangerous."
A police statement did not identify Ms. Brooks by name but said a 43-year-old woman had been detained for questioning by officers investigating both the phone-hacking scandal and payments made to corrupt police officers. A News International official, speaking on the condition of anonymity, confirmed that the woman detained was Ms. Brooks.
The British Press Association news agency said Ms. Brooks was arrested by appointment at a London police station around midday and remained in custody.
But Mr. Wilson, her lawyer, said the arrest came as a complete surprise to Ms. Brooks, who this morning believed she was attending a prearranged and voluntary sit-down session to aid police in their inquiries. When she showed up, she was arrested, he said.
News International executives expected Ms. Brooks would be answering police questions as a witness. "It was a surprise to all of us here that she was arrested today," said a senior News International official, who spoke on condition of anonymity. "When she resigned on Friday, we were not aware that she would be arrested by the police. And we did not know on Friday that she had made voluntary arrangements to go in and see the police."
Daisy Dunlop, a News International spokeswoman, declined to say whether News International was paying for Ms. Brooks' legal and public relations teams.
The arrest came two days after Ms. Brooks quit as chief executive at News International, the latest maneuver as the Murdoch family struggled to contain the fallout.
It also came two days before Ms. Brooks was to join Rupert Murdoch and his son James to testify before an investigative parliamentary panel. The body is focusing on the phone-hacking scandal that has erupted in the two weeks since reports emerged that The News of the World, once the top-selling Sunday tabloid and a central part of the Murdoch outpost in Britain, ordered the hacking of the phone of a 13-year-old girl, Milly Dowler, who was abducted and murdered. The case provoked huge public outrage and sympathy.
The arrest brought police scrutiny ever closer to the family that controls News International.
"The water is now lapping around the ankles of the Murdoch family," said Chris Bryant, a Labour parliamentarian who has taken legal action against The News of the World because he suspects his phone was hacked. On Sunday, for the second successive day, News International placed full-page advertisements in major newspapers promising full cooperation with the police. "There are no excuses and should be no place to hide," the advertisements said.
Ms. Brooks was editor of The News of the World at the time of Milly Dowler's abduction but has denied knowledge of the phone hacking. In response to the crisis, the Murdoch family closed The News of the World and withdrew a $12 billion bid to assume control of Britain's biggest satellite broadcaster, British Sky Broadcasting.
Mr. Murdoch and his family still own the top-selling daily tabloid, The Sun, as well as The Times of London and The Sunday Times of London. He also has a 39 percent stake in British Sky Broadcasting.
Referring to Mr. Murdoch, Mr. Miliband, the opposition leader, told The Observer newspaper on Sunday that the "amount of power in one person's hands has clearly led to abuses of power in his organization." He called the concentration of media ownership in Mr. Murdoch's hands "unhealthy."
Since the scandal erupted, the British police force - itself under fire for its close relationship with News International - has arrested five former editors from News International, including Ms. Brooks, whose high-flying career included spells as editor of The Sun and The News of the World.
In 2003, she became the first woman to edit The Sun, a tabloid that claimed its influence was so great that it could sway the outcome of national elections. Indeed, both Tony Blair in 1997 and the Conservative Party leader David Cameron in 2010 were backed by The Sun when they came to power.
Her arrest followed the earlier detention of Andy Coulson, another former editor of The News of the World who later became Mr. Cameron's director of communications - a job he quit in January as the hacking scandal grew more serious.
Both Ms. Brooks and Mr. Coulson enjoyed friendly relations with Mr. Cameron, prompting the opposition Labour Party to question his judgment. Ms. Brooks was also reported to be close to Mr. Blair during his time in office from 1997 to 2007.
Ms. Brooks was the most senior of former Murdoch employees to be arrested. She had been depicted as particularly close to Rupert Murdoch, who once described her as "a great campaigning editor who has worked her way up through the company with an energy and enthusiasm that reflects true passion for newspapers and an understanding of the crucial contribution that independence journalism makes to society."
At a public hearing in 2003, however, Ms. Brooks seemed to admit to lawmakers that journalists on her staffs had paid the police for information. That statement - which she later sought to retract - seems likely to offer the police a potentially rich seam of questioning.
Police officers themselves are under scrutiny, the most senior of them Sir Paul Stephenson, the head of Scotland Yard, as London's Metropolitan Police is known. According to news reports, Mr. Stephenson hired a former News of the World executive, Neil Wallis, as a public relations adviser. Mr. Wallis was himself arrested for questioning last week.
Mr. Wallis also worked for a spa where Mr. Stephenson was treated for five weeks while recovering from a fractured leg earlier this year, the Press Association said. But Scotland Yard said Mr. Stephenson did not know that Mr. Wallis worked there. Indeed, Scotland Yard said, Mr. Stephenson's stay at the spa, a £12,000 expense (about $17,000) that was extended to him as a gift, was arranged by a friend, who was the managing director of the establishment.
Scotland Yard said the police paid for Mr. Stephenson's "intensive physiotherapy" to hasten his return to work.
Referring to the arrest of Ms. Brooks, Chris Bryant, the Labour legislator, said in a telephone interview, "It looks as though the Metropolitan Police are now doing the investigation they should have been doing years ago."
"Being of a suspicious mind," Mr. Bryant added, "I do find it odd that they should arrest her now by appointment," suggesting that the timing might jeopardize parliamentary questioning scheduled for Tuesday.
Keith Vaz, the chairman of the home affairs committee, which is scheduled to hear from Mr. Stephenson at a separate hearing on Tuesday, said, "I am very surprised at this dramatic development as it comes only 48 hours after Rebekah Brooks resigned and 48 hours before she is scheduled to give evidence."
Mr. Vaz said the arrest would make difficult if not impossible for Ms. Brooks to answer any substantive questions posed by lawmakers this week.
The Met growing a spine and arresting The Man Who Sold The World & son, swiftly followed by a rigorous F.B.I. investigation.