LONDON (AP) -- Summoned by lawmakers to answer for a phone hacking and bribery scandal at one of his tabloids, Rupert Murdoch said he was humbled and ashamed Tuesday but accepted no responsibility for wrongdoing as a widening investigation threatened to ensnare Britain's prime minister.

In a three-hour grilling, the 80-year-old media tycoon insisted he was at fault only for trusting the wrong people at the now-defunct News of the World, and noted that the paper made up a tiny portion of his vast media empire.

The scandal has rocked Murdoch's News Corp. and embroiled Britain's top police, many journalists and politicians. Prime Minister David Cameron cut short his Africa trip to appear before a special parliamentary question session on Wednesday.

Murdoch appeared confused and flustered in the beginning of Tuesday's parliamentary hearing, turning frequently to his son James for answers. But he soon regained his trademark cool.

He said he had known nothing of allegations that staff at the News of the World tabloid hacked into mobile phones and bribed police to get information on celebrities, politicians and crime victims, and that he never would have approved such ``horrible invasions'' of privacy.

In the face of lawmakers' suggestions that his organization encouraged such behavior, he was unflappable -- even after a protester rushed at him in the middle of the hearing.

He stayed seated when the man tried to throw a foam pie at him. A News Corp. attorney partially blocked the attack and Murdoch's 42-year-old wife slapped the prankster. After the protester was arrested, the billionaire simply shed his splattered suit jacket and continued answering questions.

The scandal has captivated audiences from America to Murdoch's native Australia, and there's more to come -- only a fraction of the nearly 4,000 people whose information was hacked are known and the police investigation appears to be widening. Murdoch has already shut the News of the World, given up on buying a major British satellite television company and accepted the resignations of two top executives because of the scandal.

He said he had no plans to resign but expressed contrition on behalf of News Corp.'s British newspaper division, News International.

``This is the most humble day of my career,'' said Murdoch, a man once so politically powerful in Britain that former Prime Minister Tony Blair flew halfway around the world to secure his support as he launched the Labour Party's bid for power in 1995.

The scandal began as a blip in 2005, when the News of the World published a story about Prince William suffering a knee injury. Royal officials became suspicious about the closely held data and alerted police. An inquiry led to one of the paper's reporters and a private investigator being jailed for intercepting communications.

The Guardian newspaper then found out that Murdoch's papers had paid out more than $1.6 million (1 million pounds) to settle lawsuits involving allegations of eavesdropping on phone messages. The scandal became a crisis for News International this month with the revelation that the News of the World had hacked into the phone of a 13-year-old murder victim, Milly Dowler, in hopes of getting material for news stories.

Occasionally punctuating remarks by slapping his hands down on the desk, Murdoch said he was ``shocked, appalled and ashamed'' at the hacking of Dowler's phone but he rejected that criminality had been endemic at the tabloid. He also said he had seen no evidence that victims of the Sept. 11, 2001, terror attack were hacked -- an allegation the FBI is looking into.

Murdoch said he had not been informed that his company had paid out big sums -- 700,000 pounds ($1.1 million) in one case -- to settle lawsuits by phone hacking victims.

Murdoch noted that News of the World represented less than 1 percent of his global media empire, which also includes the Fox TV network, 20th Century Fox film studio, The Wall Street Journal, the New York Post and three major British newspapers. He said he spoke to the News of the World's editor only around once a month.

Blame, he said, rested with ``the people I trusted ... and then, maybe, the people they trusted.''

News International CEO Rebekah Brooks and Wall Street Journal publisher Les Hinton, former executive chairman of News International, have resigned, though Murdoch said he did not blame either of them for the scandal. Brooks and Cameron's former communications chief, Andy Coulson -- a former editor at News of the World -- are among several people who have been arrested in the scandal, though no one has been charged with a crime.

James Murdoch, 38, apologized for the scandal, telling British lawmakers that ``these actions do not live up to the standards our company aspires to.''

The younger Murdoch said the company acted as swiftly and transparently as possible. Rupert Murdoch acknowledged, however, that he did not investigate after Brooks, former editor of the News of the World, told parliament years ago that the paper had paid police officers for information.

The value of News Corp. added around $2 billion while the Murdochs were being grilled, trading 5.3 percent higher at $15.74. The stock has taken a battering over the past couple of weeks, shedding around 17 percent of its value, or around $8 billion.

Brooks testified after the Murdochs. All three agreed to provide evidence only after they were summoned.

Brooks described allegations of voicemail intercepts of crime victims as ``pretty horrific and abhorrent.'' She said had been told by News of the World employees that allegations of phone hacking by News of the World journalists were untrue, and that she realized the gravity of the situation only when she saw documents lodged in a civil damages case by actress Sienna Miller last year.

Brooks also said she had never knowingly sanctioned a payoff to a police officer.