The official, Sir Paul Stephenson, commissioner of the Metropolitan Police Service, commonly known as the Met or Scotland Yard, said that he had decided to step down because “the ongoing speculation and accusations relating to the Met’s links with News International at a senior level” had made it difficult for him to do his job.

But he said that he had done nothing wrong and that he would not “lose sleep over my personal integrity.” He also said that because he had not been involved in the original phone-hacking investigation, he had had no idea that Neil Wallis, a former News of the World deputy editor who had become a public-relations consultant for the police after leaving the paper, was himself suspected of phone hacking.

Mr. Wallis, 60, was arrested last Thursday.

The commissioner’s resignation came as the London political establishment was still digesting the stunning news about the arrest of Ms. Brooks — who apparently was surprised herself. A consummate networker who has always been assiduously courted by politicians and whose friends include Prime Minister David Cameron, Ms. Brooks, 43, is the 10th and by far the most powerful person to be arrested so far in the phone-hacking scandal.

Her arrest is bound to be particularly wounding to Mr. Murdoch, who, asked early last week to identify his chief priority in the affair, pointed to Ms. Brooks and said, “This one.”

Ms. Brooks has not yet been formally charged, but it is significant that she is being questioned in connection with two separate investigations. One, called Operation Weeting, is examining allegations of widespread phone hacking at the News of the World, the tabloid at the center of the scandal, where Ms. Brooks was editor from 2000 to 2003. The other is Operation Elveden, which is looking into more serious charges that News International editors paid police officers for information.

Ms. Brooks has always maintained that she was unaware of wrongdoing at The News of the World, which was summarily closed by Mr. Murdoch a week ago in an unsuccessful damage-control exercise. But the tide rose against her, and on Friday she resigned, saying in a statement that her presence was “detracting attention” from the company.

The arrest was a shock to the News Corporation, the parent company of News International, and the other properties in Mr. Murdoch’s media empire, which is reeling from the traumas of last week: the forced withdrawal of its cherished $12 billion takeover bid for British Sky Broadcasting and the resignations not only of Ms. Brooks but also of Les Hinton, a longtime Murdoch ally and friend who was the chairman of Dow Jones and the publisher of The Wall Street Journal. Speaking of Ms. Brooks, an official at News International said: “When she resigned on Friday, we were not aware that she would be arrested by the police.” Another person briefed on the News Corporation’s plans said that on Friday, when the company was preparing to announce her exit and the departure in New York of Mr. Hinton, the possibility of her arrest was not discussed.

Investor unease about the scandal appeared to be affecting News Corporation shares, which were down nearly 6 percent in early Monday trading on the Australian exchange in Sydney.

Until Ms. Brooks arrived at a London police station by prearranged appointment on Sunday, she believed she would merely be helping the police as a witness, her spokesman said.

“She was very surprised, I think, to then be arrested,” said the spokesman, David Wilson, chairman of the Bell Pottinger public relations firm. Mr. Wilson said it all happened so quickly that both her lawyer and he were brought in to handle her case over the weekend.

Ms. Brooks was arrested “under caution,” he said, meaning that she was read her rights and treated as a suspect. “She maintains her innocence, absolutely,” he said. She was released on bail after about 12 hours in police custody, news services reported.For months, Ms. Brooks had been willing to talk to the police but had been rebuffed, Mr. Wilson said. “As recently as last week, she was told she wasn’t required to do so and she wasn’t on their radar.”

Jo Becker and Ravi Somaiya contributed reporting from London, and Jeremy W. Peters from New York.

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