Kenyan Vice-President William Ruto left Nairobi to fly to The Hague Monday, a day before the start of his crimes against humanity trial at the International Criminal Court.

Ruto, 46, charged with masterminding deadly post-election violence five years ago, is the most senior politician to date to face ICC judges, and his trial will start just days after Kenyan MPs voted to abandon the court in a world first.

He shook hands with supporters and with a wide grin waved to the small crowd at the airport as he left, but made no comment.

Ruto is accused by the ICC of masterminding some of the 2007-2008 post-election unrest that left at least 1,100 dead and more than 600,000 homeless.

Fellow accused Joshua arap Sang, a radio boss accused of inciting and helping coordinate attacks during the violence, is already in The Hague, after flying there over the weekend.

Both will plead not guilty.

On Sunday, Ruto prayed in church alongside President Uhuru Kenyatta, also accused of organising a campaign of murder, rape, persecution and deportation, whose ICC trial begins on 12 November.

However, Kenyatta on Sunday said that he will not allow both leaders to be away from Kenya at the same time.

"We will work with ICC, and we have always promised to do this," Kenyatta told a rally of supporters. "But it must understand that Kenya has a constitution, and Ruto and myself won't be away at the same time."

Kenyatta, who will also plead not guilty, said the "charges are false and will be dismissed."

"The plotters of this scheme will be named and shamed. The victory won't be Ruto's, Sang's or mine, but for Kenya," he said to cheering crowds.

Kenya's 2007 elections were marred by allegations of vote rigging, but what began as political riots quickly spiralled into a wave of ethnic killings and reprisal attacks, the worst violence in the country since independence in 1963.

Human Rights Watch on Monday called the case the "first real effort" to look at responsibility for the killings.

"For decades those who have turned Kenya's elections into bloodbaths have gotten away with murder," HRW Africa director Daniel Bekele said in a statement.

"This ICC trial tackles an impunity crisis in the country and offers a chance for justice denied to Kenyans by their own government."

But there is also concern in Kenya that the trials could reopen old wounds and undo efforts by communities who once fought each other in deadly battles.

Law Society of Kenya chairman Eric Mutua on Monday warned that "heavy political statements and debates on the ICC process may tear the country apart," in a statement urging "leaders and the public to exercise caution as justice takes its course."