At least two people have been shot dead in Kenya during protests by opposition supporters against the re-election of President Uhuru Kenyatta, officials said.

“We have one person killed and four others admitted in hospital with gunshot injuries,” said Dr Ojwang Lusi, the regional health chief in western Kisumu county.

In the south-western town of Siaya, a police officer said a man had been shot dead in protests, but “we have not managed to collect the body … because of resistance from protesters”.

Protests flared in opposition strongholds in Nairobi and Kisumu after the election commission declared Kenyatta the victor in a hotly disputed vote. Supporters of his rival, Raila Odinga, claim the vote was rigged.

After late night looting and riots, anger remained high on Saturday morning, with running battles in the Mathare and Kibera slums of the capital.

There were also pro-Kenyatta celebrations, although a senior traffic police officer said four people were killed when they were hit by vehicles while marking his victory.

After an acrimonious campaign, Kenyatta used his re-election speech to reach out to Odinga and his supporters on Friday night, saying he wanted to work with them “in the service of Kenya”.

The 55-year-old, who has been in power since 2013, called on his compatriots to “remember that we are brothers and sisters” and to shun division.

In recent days, opposition officials have described the election results as a fraud and claimed that Odinga, the 72-year-old Nasa leader, was the legitimate winner.

Kenyans had endured a tense three-day wait while a definitive count was done of the results from more than 40,000 polling stations.

In 2007, Odinga’s claims of vote rigging prompted rioting and retaliation by security forces, which tipped the country into its worst crisis for decades with about 1,200 people being killed.

A series of election observers and western officials have advised the losers in this year’s election to accept defeat. John Kerry, the former US secretary of state, called on political candidates and parties to stay within the law to resolve any disputes.

“The judicial process, the judicial system of Kenya and the election laws themselves make full and adequate provision for accountability in this election,” said Kerry, an election observer for the Carter Center. “The streets do not.”

Opposition politicians accused the observers of “rushing to judgment” and colluding with the government.