By Michael Martina and Benjamin Kang Lim

BEIJING (Reuters) - China's Communist Party gave President Xi Jinping the title of "core" leader on Thursday, putting him on par with past strongmen like Mao Zedong and Deng Xiaoping, but it signaled his power would not be absolute.

A lengthy communique released by the party following a four-day, closed-door meeting of senior officials in Beijing stressed maintaining the importance of collective leadership.

The collective leadership system "must always be followed and should not be violated by any organization or individual under any circumstance or for any reason", it said.

But all party members should "closely unite around the Central Committee with Comrade Xi Jinping as the core", said the document, released through state media.

The "core" leader title marks a significant strengthening of Xi's position ahead of a key party congress next year, at which a new Standing Committee, the pinnacle of power in China, will be constituted.

Since assuming office almost four years ago, Xi has rapidly consolidated power, including heading a group leading economic reform and appointing himself commander-in-chief of the military, though as head of the Central Military Commission he already controls the armed forces.

While head of the party, the military and the state, Xi had not previously been given the title "core".

Deng coined the phrase "core" leader. He said Mao, himself and Jiang Zemin were core leaders, meaning they had almost absolute authority and should not be questioned.

The once-every-five-years congress will be held in the second half of 2017 and Xi will be looking to stack the Standing Committee with as many of his own people as possible.

Zhang Lifan, a Beijing-based political commentator, said now that Xi was the "core", things should go more smoothly for him at next year's congress. But he will have more on the line, given his increased responsibility to answer economic and social problems facing the leadership.

"If the economy continues to go downhill and the rifts in society become more serious, the responsibility of the core is greater," Zhang said. "Your relative power and authority are greater, everyone is deferring to you. But they will be watching to see if your leadership is good or bad."

STILL A YEAR TO GO

Xi's immediate predecessor, Hu Jintao, was never called the "core".

An unofficial campaign to name Xi the "core" has been underway this year, with about two-thirds of provincial leaders referring to him as such in speeches, according to figures compiled by Reuters, before the plenum formally accorded him the title.

Steve Tsang, professor of Contemporary Chinese Studies at the University of Nottingham in Britain, said while Xi is in a strong position, there was still a year to go before the congress.

"There's still a lot of unanswered questions. Will his successor be named? Will Xi get a third term?"

Judging by recent past precedent, Xi should step down at the 2022 congress after a decade at the top, but speculation in leadership circles has swirled that he may try and stay on, perhaps giving up the post as president but remaining as party leader, the more senior of the posts.

In its turgid statement, the party also announced changes to the Rules on Intra-Party Political Life, first introduced in 1980 to prevent any cult of personality after Mao's rule plunged the country into anarchy during the 1966-1976 Cultural Revolution. But it gave no detailed explanation on how the rules would change.

While a cult of personality had begun to form around Xi, he has moved to stop practices including adoring songs on the internet and references to him in state media as "Uncle Xi", sources with ties to the leadership say.

"Publicity regarding leaders should be based purely on facts and boasting should be banned," the party said in its statement.

New rules on "Intra-Party Supervision" - code for how to fight corruption - were also approved at the plenum, but few details were given either.

The party will end the buying and selling of official posts, a common problem, and the selection and appointment of officials should not be "contaminated by outside interference or misconduct", the party said.

Xi has waged battle on corruption, jailing dozens of senior officials, warning the problem is so severe it could affect the party's grip on power.

According to past practice, a longer statement on the plenum, giving more details on what was agreed, should be released sometime in the next two weeks.

(Additional reporting by Sue-Lin Wong; Writing by Ben Blanchard; Editing by Raju Gopalakrishnan)