The election is shaping up to be the closest ever in east Africa's biggest economy, with the latest opinion polls showing Kibaki trailing his former ally Raila Odinga.

Local media houses have been keen to broadcast both men in a verbal showdown that newspapers have dubbed "The Grand Rumble". Odinga was keen, but on Tuesday Kibaki's office said no.

Odinga's challenge was a "late conversion" to the idea the campaigns should be focused on issues, it said in a statement, but the opposition leader had already used the media to express his "biased sentiments" against the president's policies.

"President Kibaki will, therefore, not be drawn into a debate that will not add value to the campaign and believes that any presidential debate must be focused and objective and address pertinent issues that affect Kenyans and not on narrow partisan agendas that only serve individual purposes," it said.

Odinga's challenge, it added, could only amount to him seeking a platform to entrench his "parochial and biased ideas".

On Monday, Odinga's Orange Democratic Movement accused the government of bribing voters to back Kibaki and warned of chaos and regional instability if the election was rigged. A spokesman for Kibaki's Party of National Unity rejected that.

But the campaigns have already been marred by bloodshed, most of it during internal party primaries last month to pick candidates for parliamentary seats.

Thousands of Kenyans have had to flee their homes because of political violence, particularly around Mount Elgon and in part of Rift Valley province, and 16 people have died.