 Some say there has been "ethnic cleansing" in Kenya |
The police say that David Too was killed in a domestic dispute unrelated to the political crisis but the opposition call it an "assassination".
A local police chief says Mr Too was having an affair with the girlfriend of the policeman, who shot them both.
The killing has led to the postponement of peace talks in the capital, Nairobi.
The first substantive talks between government and opposition began earlier on Thursday.
"We have postponed this afternoon's session and we will work all day tomorrow so that the leaders can attend to urgent matters and call their constituents," said former UN Secretary General Kofi Annan, who is mediating the talks.
'Plot'
Angry crowds have marched on the Eldoret police station, where they were dispersed by officers firing shots into the air, reports the AP news agency.
Trucks have been set on fire outside the town, while members of President Mwai Kibaki's Kikuyu community have been fleeing the Rift Valley town, seen as an opposition stronghold.
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A resident of Mr Too's constituency of Kericho in Western province told the BBC there was also unrest there.
The police are struggling to contain crowds of angry youths, who have set several buildings on fire, he said.
An opposition MP was shot dead on Tuesday, sparking violence in Nairobi slums.
"The second killing of an MP belonging to Orange Democratic Movement is part of a plot to reduce our majority in parliament," ODM leader Raila Odinga told the AFP news agency.
While Mr Kibaki was declared the victor of the presidential election, the ODM won twice as many seats as Mr Kibaki's party in the parliamentary race held on the same day.
'Love triangle'
Eldoret Deputy Police Chief Gabriel Kuya told AP that the traffic policeman followed a car carrying Mr Too and the woman on his motorbike.
"He drove toward the side of the woman and shot her in the stomach twice. Her partner [Mr Too] pleaded with the officer not to kill her but he turned his pistol on him instead, hitting him four times in the head," he said.
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More than 850 people have been killed since the disputed presidential poll.
Another 250,000 have fled their homes in what used to be one of East Africa's most stable countries.
The Rift Valley has seen some of the worst violence since the 27 December election.
On Wednesday, the top US official for Africa, Jendayi Frazer, said there had been "ethnic cleansing" in the region.
