* Veteran leader denies prostate cancer reports
* Succession battle hotting up in ZANU-PF
* Critics say Mugabe dropping ball on economy
By Cris Chinaka
HARARE, Feb 21 (Reuters) - Zimbabwean President Robert Mugabe turned 90 on Friday, far from home on a medical trip that has intensified concerns about his health and stoked a simmering succession battle in a country under his thumb for more than three decades.
When Mugabe left for Singapore this week, spokesman George Charamba maintained the official denial of reports that Africa's oldest president was suffering from prostate cancer, saying the trip was for a "routine and long-planned" cataract operation.
Charamba assured Zimbabweans that Mugabe would be back home for his official birthday celebrations on Sunday, but the timing of the trip - just days before the milestone - has fired up speculation that Mugabe's health is failing.
In an interview with state television, his traditional mouthpiece, Mugabe claimed to be "as fit as a fiddle" but his slow speech and puffy appearance only made tongues wag harder.
"I know he's a very old man and we should not expect him to look like a 25-year boxing champion or rugby player, but it's exactly for that reason that he should be retiring," said one 50-year-old currency trader after seeing the interview.
As with many Zimbabweans discussing a leader accused of staying in power by force, he did not want his name to appear in print.
A June 2008 U.S. diplomatic cable published by Wikileaks said Mugabe had prostate cancer that had spread to other organs, and had been urged by his physician to step down from office.
But Mugabe went on to win another five years in power at an election last year that he says will allow him to quell factional fighting in his ZANU-PF party over who will one day fill his shoes.
"When the day comes and I retire, yes, sure, the day will come. But I do not want to leave my party in tatters. I want to leave it intact," he said in the state television interview.
JOICE VS. THE CROCODILE
Joice Mujuru, Mugabe's deputy in both the government and the ZANU-PF party, is leading the race to replace him, analysts say, closely followed by Justice Minister Emmerson Mnangagwa - a powerful former defence minister nicknamed 'The Crocodile'.
If Mugabe dies without resolving the succession issue, there are fears ZANU-PF could implode in a factional fight with the potential to suck in the military.
The constitution says if the president dies mid-term, the ruling party will elect a successor. ZANU-PF's charter says this must be done at a party congress where, technically, any aspiring leader can contest.
Mujuru has consolidated her support base with newly-elected provincial executives but she needs Mugabe's open endorsement to overcome reservations in the top ranks of the army about her capacity to lead, analysts say.
"It's fair to say nothing is given, that there will be no coronation but a contest for power," said Eldred Masunugure, a political science professor at the University of Zimbabwe.
"It is also very likely that this fight will get into the last round in Mugabe's absence," he said.
Besides fears of outright political unrest, many of the southern African nation's 13 million people worry that Mugabe and ZANU-PF big-hitters are preoccupied with succession at the expense of an economy desperate for revival.
In an commentary titled "Echoes of the end of an era," the private Zimbabwe Independent newspaper said Mugabe was struggling to address serious problems, not least capital flight since last year's election and a plunge in business confidence.
"Zimbabwe now seems to be running on auto pilot," the paper said. "There appears to be some loss of control." (Editing by Ed Cropley and Andrew Heavens)