Supporters of opposition presidential candidate Raila Odinga allegedly threw stones at plainclothes officers from Nairobi who arrived in Migori, The Independent reported. Besides the three killed, the government said another nine were in critical condition, the British newspaper reported.

Odinga, who polls indicated was neck and neck with President Mwai Kibaki going into the election, alleged thousands of police officers had been given unlimited access to polling stations to stuff ballot boxes with millions of votes for Kibaki.

"They were issued each with 1,000 pre-marked ballot papers to be stuffed by them," Odinga said. "I've appealed to President Kibaki to prevail on his agents against election rigging acts that could plunge this country into bloodshed."

The Kibaki government denounced the killings, insisting the deployment was a routine procedure to provide extra security for the election.

"The government has deployed over 65,000 security agents to all parts of the country, in keeping with the request by the Electoral Commission of Kenya, to ensure security during the voting process," said a presidential spokesman.