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A court sentenced China's former security chief to life in prison Thursday after convicting him of corruption in a secretly held trial, underscoring President Xi Jinping's anti-graft campaign and further cementing Xi's authority by burying a once-powerful political faction.

Zhou Yongkang, 72, was spared the death penalty in a decision seen as a show of leniency toward the former Politburo Standing Committee member, the highest-level politician to face court in China in more than three decades.

The dour and once-feared Zhou expressed remorse on state television in an appearance in which he looked much older and haggard, with undyed grey hair rather than the dyed-black hair he had when he was last seen publicly in late 2013.

"I accept the court verdict, and I will not appeal," Zhou told the court, with his head lowered and body slightly bowed.

The First Intermediate People's Court of Tianjin in northeastern China said it had convicted Zhou of taking massive amounts of bribes, abusing power and leaking state secrets in a trial on May 22. The court cited the latter charge as the reason why the trial was held behind closed doors and not publicly announced at the time.

However, observers have said the closed-door trial may have been arranged by the Communist Party leadership to prevent any lurid details of the party's inner workings from becoming public.

Zhou was once seen as not only a potent rival of Xi but as the center of a vast patronage network stemming from his separate stints as an executive in the state-owned oil industry, party boss in the southwestern province of Sichuan and head of state security.

"The case involves senior folks from the petroleum industry, the state security apparatus with many corruption scandals," said Willy Lam, an expert on China's elite politics at the Chinese University of Hong Kong. "The authorities wanted to avoid having them appear in the public."

Lam said Zhou had been expected to receive a suspended death sentence, and that the life sentence indicated that Xi wanted to compromise and not antagonize any party members who had been allied with the former security czar.

"President Xi does not want to make too many enemies," Lam said. "Xi has won the battle and he does not want to go too far."

Authorities also did not want to see Zhou put up a feisty fight in an open court, Lam said.

His expression of contrition on state TV also helped quell the notion of continued factional fighting at the party's uppermost echelon.

Zhou was sentenced to life in prison for accepting what the court said were "particularly huge bribes." He was given lesser terms on the abuse of power and state secrets charges, and was ordered to serve his sentences concurrently. Zhou's sentence also mandated the seizure of all his personal assets.

According to the court, Zhou received, directly and indirectly, a total of 130 million yuan ($21 million) in bribes and used his influence to allow others to realize 2.1 billion ($343 million) in profits on business dealings that caused 1.4 billion ($229 million) in losses for the state treasury — presumably through the sale of government assets at below cost.

Zhou's actions "inflicted enormous damage to public finances and the interests of the nation and the people," the court said in an explanation of the verdict on its website.

While the charges potentially mandated a death sentence, it said Zhou received leniency after confessing and showing repentance and ordering his relatives to hand over the majority of their ill-gotten gains.

Although the charges of abuse of power and leaking state secrets were serious, they had not resulted in any major consequences, the court said. The court said Zhou had passed state secrets to his Qigong master, a kind of spiritual mentor, though it did not detail the secrets or say what the master had done with the information.

Zhou is the highest-ranking former politician to face court since the 1981 treason trial of Mao Zedong's wife and other members of the "Gang of Four" who persecuted political opponents during the 1966-76 Cultural Revolution.

Zhou had been under investigation since late 2013, and his former allies in government and the oil industry also were scrutinized.

Zhou was once seen as untouchable, and his downfall has been touted as a flagship case for Xi's anti-corruption drive, which is supposed to target the "tigers" — or top-ranking officials — along with the "flies" — or the low-ranking ones.

But Zhang Lifan, a Beijing-based independent historian and political commentator, questioned whether Zhou was a true tiger, given that he already was retired and much of his influence had diminished.

He was a "clawless tiger," Zhang said.

Zhou spent the early part of his career in the oil industry, rising through the ranks over several decades to become the general manager of China National Petroleum Corp., one of the world's biggest energy companies, in 1996.

He was Sichuan party chief from 1999-2002. Later, he oversaw China's vast police and security apparatus.