Kenyan opposition leader Raila Odinga has claimed hackers infiltrated the database of the country's election commission and manipulated the results in what he called an "attack on our democracy".

His allegations of fraud on Wednesday followed the release of election results showing President Uhuru Kenyatta with a wide lead over Mr Odinga after votes from the vast majority of polling stations had been counted.

Mr Odinga claimed the alleged hackers "created errors" in the election commission's database and described Tuesday's election as a "fraud".

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Mr Odinga also ran against Mr Kenyatta in the 2013 vote and unsuccessfully challenged the results in court with allegations of vote-tampering.

The long-time opposition figure also ran in the 2007 election, which was followed by violence fuelled by ethnic divisions that killed more than 1,000 people.

Later, a witness said hundreds of protesters are rioting in Kisumu, a city in south western Kenya that is a stronghold of Mr Odinga.

Kenyan journalist Fred Ooko said people from the Kondele slum in Kisumu burned tyres and blocked roads in the port city on Lake Victoria, one of Kenya's largest urban centres.

Authorities have been concerned about possible violence following elections on Tuesday.