NAIROBI — Kenya held a rerun of its botched presidential election Thursday, but the opposition boycotted and clashes broke out between police and protesters, threatening further political turmoil in one of sub-Saharan Africa’s key economies.

The incumbent, Uhuru Kenyatta, was viewed as almost certain to win because of a boycott by the leading opposition candidate, Raila Odinga, who claimed it was impossible to hold a credible election.

Kenya has been seen as a maturing democracy and a key Western ally in a region troubled by war and an Islamist insurgency.

Violence was concentrated in opposition strongholds, particularly the western city of Kisumu, where hospitals reported several people injured by gunshots, including one man who died from his wounds.

By late afternoon, with word coming in that protesters had prevented voting materials from being distributed in some areas, the election commission said the vote was postponed to Saturday in four counties while security is restored.

The original August election was annulled by Kenya’s Supreme Court, which cited irregularities in the electoral process. But the process of holding a new election has been beset by problems, including the withdrawal of Odinga. Two members of the commission overseeing the election also publicly expressed doubt that the contest could be free and fair.

The rift in the country falls largely along ethnic lines, between Kenyatta’s Kikuyu tribe and Odinga’s Luo tribe, and was acutely visible across Nairobi on Thursday.

Kenyatta supporters arrived at the polls to affirm his reelection. Odinga supporters stayed home and refused to vote, or blocked access to polling stations and lobbed rocks at police, who responded with force.