(Recasts, adds details and background)

By Joe Bavier

KINSHASA, Sept 25 (Reuters) - Congo's octogenarian prime minister, Antoine Gizenga, quit on Thursday on health grounds, but his departure followed criticism that he was failing to revive the war-ravaged economy and end conflict in the east.

Gizenga, 83 next month, was appointed to head Democratic Republic of Congo's government in late 2006 by President Joseph Kabila as a reward for instructing his Unified Lumumbist Party (PALU) to back Kabila in a presidential run-off vote.

"I've decided today to present my resignation to the president," Gizenga said in a television broadcast.

He said that, although his spirit was strong, he had realised his ageing body was no longer up to the task of leading the government of the vast, mineral-rich central African state, which is still recovering from a 1998-2003 war.

During his nearly two years as prime minister, Gizenga's administration came under increasing fire from critics for not doing enough to relaunch the war-shattered economy and resolve lingering conflicts in Congo's volatile eastern borderlands.

"He tried his best, but I think it's time for him to make way for new blood," Clement Mimundele, a resident of Congo's capital, Kinshasa, told Reuters.

The prime minister's decision to step down places in doubt his PALU party's future in the government. The party holds the key ministries of the budget and mines, which has undertaken a review of the country's mining contracts.

Parties closely allied to the president already dominate both houses of parliament and most provincial assemblies.

NO LEADING FIGURE

Without a leading figure to fill Gizenga's shoes, few believe PALU will maintain the premiership, and Kabila could use the opportunity to further extend his control over Congo's political landscape, observers say.

"As I understand it, Kabila will be presented three people, possibly today, and he could make a decision on a new prime minister tonight," a Western diplomat in Kinshasa told Reuters.

Likely candidates for the job include Planning Minister Olivier Kamitatu, National Assembly President Vital Kamerhe, and Agriculture Minister Francois Joseph Mobutu Nzanga, son of former ruler Mobutu Sese Seko.

On Wednesday, Kamerhe, the head of the lower house of parliament and a presidential ally, told U.N.-backed Radio Okapi that legislators planned to summon Gizenga for a possible vote of no confidence against him.

"Democracy has done its job today," Gilbert Kiakwama, a key opposition MP who was preparing the vote of no confidence, told Reuters. "We want to see a change in how things are done. Whoever is chosen must defend the interests of the nation."

A veteran opposition figure, Gizenga had served as deputy to Congo's first prime minister, Patrice Lumumba, who was assassinated in 1961 just months after the central African nation achieved independence from Belgium.

He remained a figurehead of opposition to the kleptocratic dictatorship of the late Mobutu Sese Seko and came third in the 2006 presidential election won by Kabila that marked Congo's first democratic polls in more than four decades.

(For full Reuters Africa coverage and to have your say on the top issues, visit: http://africa.reuters.com/) (Editing by Pascal Fletcher and Andrew Dobbie)

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