* Election commission claims turnout of 60 pct
* Ouattara widely favoured to win a new five-year term
* Observers give election a clean bill of health (Adds observers' turnout estimate, candidate recognising Ouattara victory)
By Joe Bavier and Loucoumane Coulibaly
ABIDJAN, Oct 26 (Reuters) - Observers gave Ivory Coast's weekend presidential ballot a clean bill of health on Monday in a boost to runaway favourite President Alassane Ouattara, while one challenger conceded defeat before the first results were announced.
A former senior IMF official, Ouattara has led the world's top cocoa grower to a revival, after a 2011 civil war, that has made it a magnet for investment as other African economies have crumbled amid a global commodities crash.
Sunday's vote was marked by some organisational hitches, including the late arrival of materials that led the Independent Electoral Commission (CEI) to extend voting in some places by two hours. Many of the computer tablets used to verify voters' identities also failed.
PEACE CI, a platform of civil society organisations that fielded some 2,000 observers, said that minor organisational problems had not affected the credibility of the election.
"We believe the election was peaceful, transparent, credible and inclusive," said U.S. ambassador Terence McCulley, speaking for a separate U.S. observation mission that included 70 diplomats.
The commercial capital, Abidjan, was calm on Monday, though traffic was light, as the nation of about 24 million people awaited initial results from the CEI, now expected on Tuesday.
Faced with a partial opposition boycott and concern over voter apathy, turnout will be critical to legitimise Ouattara's second five-year mandate if he wins.
An early, preliminary CEI estimate of 60 percent turnout appeared to allay fears that voters had stayed home on Sunday.
However, POECI, a civil society observer platform that is carrying out a parallel vote tabulation, said its data put turnout somewhat lower at 53 percent. West African regional bloc ECOWAS's mission said it witnessed weak participation at most voting sites it visited.
Both organisations said the vote was fair and transparent.
Several opposition candidates had dropped out of the race in the days before the election, alleging it had been rigged and called upon voters to stay home. The CEI dismissed the allegations as unsubstantiated.
Simeon Konan Kouadio, one of the six candidates who remained in the race to unseat Ouattara, said his campaign team had been informed of massive fraud but offered no evidence.
In a surprise announcement, candidate Bertin Konan Kouadio conceded defeat by Ouattara even before official results began to emerge. He had earlier alleged irregularities in voting, including foreigners being allowed to cast a ballot.
"According to results in our possession, it seems clear that Alassane Ouattara is winning the majority of votes necessary for his re-election," Kouadio told journalists. "I would therefore like to offer him my congratulations."
The question of nationality has long been a burning political issue in Ivory Coast, where decades of relative prosperity have attracted Muslim migrants, largely from neighbouring Mali and Burkina Faso.
Ouattara, who was barred from seeking the presidency over what opponents said were his foreign origins before finally coming to power in 2010, told Reuters last week he would seek to strip nationality clauses from the constitution if elected. (Writing by Daniel Flynn and Joe Bavier; Editing by Ralph Boulton and Ken Wills)
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