* Deputy president goes on trial for post-election violence
* Trial sets stage for that of president in November
* Proceedings will test stability in Kenya and region
Kenya's deputy president goes on trial at the International Criminal Court on Tuesday charged with co-orchestrating a post-election bloodbath five years ago, a case that will test the stability of a country seen as vital to security in East Africa.
The trials of William Ruto and that of Kenyan President Uhuru Kenyatta, which will start in November, have split public opinion, and witness testimonies of the violence in 2008 that killed more than a thousand people could stir tension.
The cases are also a major test for prosecutors at the decade-old Hague-based ICC, who have had a low success rate and face accusations of focusing on African countries, while avoiding war crimes in other global hotspots.
Kenyatta, Ruto's former rival who became a political ally, faces similar charges of crimes against humanity.
Rival members of Kenyatta's Kikuyu and Ruto's Kalenjin tribes, wielding machetes, knives, and bows and arrows, went on the rampage after the disputed elections, butchering more than 1,200 people and driving hundreds of thousands from their homes.
Ruto and his co-accused, the broadcaster Joshua arap Sang, could face long prison terms if convicted.
The cases against Ruto and Kenyatta have become increasingly fraught politically since the two were elected under a joint Jubilee Alliance ticket in March after an election campaign in which the ICC charges the two faced played a central role.
Western leaders, who see a stable Kenya as central to the fight against militant Islam, have already found their ties with east Africa's biggest economy complicated by the charges.
Ruto, who is voluntarily obeying a summons to attend sessions, arrived in The Hague on Monday and is expected to enter a plea of "not guilty" on Tuesday.
Close to 100 Kenyan legislators plan to attend the opening weeks of the trial in a show of support.
The cases may have helped him and Kenyatta into office as campaigners rallied nationalist support by accusing the court of foreign meddling in the former British colony.
The political alliance means an immediate flareup of violence is seen as unlikely, but tensions on the ground will inevitably rise.
"There will be an immediate response in local politics once these trials start," said John Githongo, a former government anti-corruption official turned whistleblower and human rights activist.
"Last time the politicians managed to turn it around for alliance building and it worked extremely well. However, invariably, once the evidence starts coming out, it will bring tension," he said.
The horrors of the election violence shattered Kenya's reputation as one of Africa's most stable democracies and dealt the economy a heavy blow from which it is only now recovering.
Anger over the charges culminated last week in a vote in the country's parliament calling for Kenya to withdraw from the international court's jurisdiction.
Kenyatta further fuelled criticism of the court on Monday when he threatened to suspend cooperation with the ICC if he and his deputy were summoned simultaneously, leaving the country without a head of state in residence.
Judges said the cases would alternate at one-month intervals. Even if Kenya does quit the court, trials already under way will continue.
Fatou Bensouda, the court's prosecutor, rejected claims of meddling, saying that the cases before the court related purely to the 2008 violence and those accused of it.
"Contrary to what has now become a rallying call for those who do not wish to see justice for victims of post-election violence, our cases have never been against the people of Kenya or against any tribe in Kenya," she said on Monday.
Some onlookers fear the proceedings have the possibility of re-igniting the rivalry.
"This case has the potential to bring a bit of tension between the two communities that fought," said political scientist Mutahi Ngunyi.
Both sides have been accused of intimidating witnesses, allegations they deny.
Karim Khan, Ruto's lawyer, said such accusations were designed to distract attention from a fundamentally weak prosecution case.
"This case will fall apart in the end. But it will fall apart because of lack of evidence because of the deficient investigations conducted, and not for any other reason."