By Ben Blanchard and Benjamin Kang Lim

BEIJING (Reuters) - China sentenced its powerful former domestic security chief Zhou Yongkang to life in jail on Thursday, after he was found guilty at a secret trial of bribery, leaking state secrets and abuse of power, in China's most sensational graft scandal in 70 years.

Zhou, who was formally charged in April, was tried in the northern city of Tianjin on May 22. He admitted his guilt and decided not to appeal against the verdict, state media said.

Zhou, 72, is the most senior Chinese official to be ensnared in a graft scandal since the Communist Party swept to power in 1949. The decision to try Zhou underscores President Xi Jinping's pledge to fight corruption at the highest levels.

"I submit myself to the verdict of the court, and I do not appeal," Zhou told the court, in comments carried on state television's main evening news.

"I recognize the facts of my breaking the law, which has caused great losses to the party. I again admit my guilt and am penitent," a white-haired Zhou, who had not been seen in public since October 2013, added.

One source with the direct knowledge of the situation told Reuters that Zhou was guarded by soldiers rather than members of the police force he used to command.

"He was cooperative during interrogations," the source said, speaking on condition of anonymity. "His attitude was good."

In ordering the investigation into Zhou, Xi broke with an unwritten understanding that members of the Politburo Standing Committee would not come under such scrutiny after retirement.

Zhou's alleged crimes took place over decades, including when he was deputy general manager of China National Petroleum Corporation (CNPC), party boss in southwestern Sichuan province, minister of public security and a member of the Politburo Standing Committee, according to the initial indictment.

Zhou's wife and son, who testified via video link, took 129 million yuan ($20.78 million) in money and property, and then told Zhou after they had taken the bribes, the court found.

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CNPC's former head Jiang Jiemin also testified. Jiang, a former close associate of Zhou, went on trial in April accused of corruption, but has yet to be sentenced.

Jiang, as well a former deputy Sichuan party boss Li Chuncheng, were told by Zhou to assist in the business activities of others, helping them to illegally obtain about 2.14 billion yuan, the court found.

The page-and-a-half statement published by the official Xinhua news agency gave brief but tantalizing details of the trial, though it did not elaborate on the nature of the state secrets he leaked.

MYSTIC LINK

Zhou handed over six secret documents from his office to a person named Cao Yongzheng, Xinhua said. Respected Chinese business magazine Caixin has previously identified Cao as a mystic.

The government had previously said the trial would be open.

While Xi, who has pledged to go after powerful "tigers" as well as lowly "flies", will continue to press home his corruption fight, his government has other more pressing problems, such as the slowing economy and need to push through painful reforms.

"I think we still cannot talk about him having achieved an overwhelming victory," said Zhang Lifan, a Beijing-based political commentator.

"In the past couple months, the crackdown on tigers has obviously slowed. Recently it has just been flies."

Zhou was a member of the Politburo Standing Committee - China's apex of power - and held the post of security tsar until he retired in 2012.

Sources with ties to the Chinese leadership have previously told Reuters that Xi has been determined to bring down Zhou for allegedly plotting appointments to retain influence ahead of the 18th Party Congress in November 2012, when Xi took over the party.

Zhou also ordered the bugging of the telephones of top leaders, the sources have said.

The government's corruption fight has extended to almost every corner of the country, including powerful state-owned companies that dominate sectors of the economy such as energy, banking and telecommunications.

Zhou joined the Politburo Standing Committee in 2007 while also heading the central Political and Legal Affairs Committee, a sprawling body that oversees law and order policy. The security apparatus he ran expanded during his watch and consumed a budget that exceeded the official figure for military spending. He quickly earned the enmity of Chinese dissidents.

Retired legislators and lawyers have said many of the previous abuses to the rule of law in China can be attributed to Zhou, who expanded his role into one of the most powerful and controversial fiefdoms in the one-party government.

($1 = 6.2065 Chinese yuan)

(Additional reporting by Michael Martina; Editing by Nick Macfie and Alex Richardson)