NAIROBI — Kenya held a rerun of its botched presidential election on 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 the 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 morning.

Kenyatta supporters arrived at the polls to affirm his reelection. Odinga supporters either stayed home and refused to vote, or, in a few tense parts of the city, blocked access to polling stations and lobbed rocks at police, who responded with force.

‘‘I am voting for a peaceful country, and I am voting for Uhuru,’’ said Selly Bahati. ‘‘But it’s true that we are divided by tribe, and I don’t know what’s going to happen next.’’ She said her business selling shoes in Nairobi has declined by 50 percent during the last few months of political instability.

Bahati arrived at an empty polling center in Kibera, Nairobi’s largest slum. While she stood outside, men screamed at her.

‘‘What are you doing there?’’ one man yelled.

Deeper in the slum, in a more fiercely pro-Odinga area, running battles between police and protesters went on for hours. Police chased young men through the slum’s narrow streets, and one protester said at least one person was shot and taken to a local hospital.

When local men spotted a bus carrying ballot papers, they shouted at the police escorting it, arguing that there was no reason to supply voting materials in a neighborhood where residents supported a candidate who had withdrawn from the election.

‘‘Why are you bringing the papers here?’’ Vincent Omondi yelled at one officer.

In the opposition stronghold of Kisumu, media reported that no polling stations had opened and that police fired tear gas and live rounds at stone-throwing young supporters of Odinga.

Kenyatta, however, told reporters as he voted in his home town of Gatunda that ‘‘90 percent of the country is calm, is peaceful.’’

A police statement backed up Kenyatta’s assertion, reporting violence in only five of Kenya’s 47 counties.

It remains unclear how Odinga’s supporters will respond once Kenyatta’s reelection is formally declared. For his part, Odinga has suggested that he will transform his party into a ‘‘resistance movement’’ aimed at obstructing the next government. He called the probability of Kenyatta’s reelection a ‘‘coup d’état’’ and the election a ‘‘day of infamy.’’