Uhuru Kenyatta, Kenya's finance minister and son of its founder president, must stand trial at the International Criminal Court for directing a militia to murder and rape after a disputed 2007 election, ICC judges ruled on Monday.
Three more Kenyans, including former minister William Ruto, must also face trial for crimes against humanity during post-election violence that killed at least 1,220 people.
The decision by the Hague-based court is likely to have far-reaching consequences for the biggest economy in east Africa where a political elite have long been considered almost above the law.
Both Kenyatta, the country's deputy prime minister who was ranked Kenya's richest man by Forbes magazine, and Ruto, a former higher education minister, plan to run for president in an election due by March next year at the latest.
Kenyatta said he was innocent of all accusations despite the court's ruling. Kenyatta said on Sunday he would run for the presidency whatever the ICC outcome.
Ruto said he found the ruling strange, would appeal against it and would run for president regardless.
President Mwai Kibaki asked Kenyans to remain calm and named a legal team to advise him on what steps to take.
Anti-corruption campaigner John Githongo told Reuters: It's a great thing in terms of the fight against impunity in the country for those who have been found to have a case to answer.
It means they have joined an exclusive, small club of individuals who have had questions to answer before the entire world with regard to crimes against humanity ... Ratko Mladic, Charles Taylor ... That's not polite company at all.
Some analysts said Kenyatta would come under pressure to quit as finance minister and a deputy prime minister and that his and Ruto's presidential campaigns could suffer.
While they would dearly love to fight on, the sheer practicalities of facing trial may make it impossible to run, not to speak of the major question marks over their integrity in the eyes of the electorate and their political backers, said Patrick Mair, an analyst at Control Risks.
JUSTICE HAS BEEN DONE
The ruling could lead to demonstrations in Kenya, although there were no immediate outbreaks of protest in Kenyatta's constituency, nor in Ruto's Rift Valley powerbase.
There are two Kenyan cases at the ICC, split broadly between the ethnic Kalenjin and Kikuyu camps involved in much of the violence. Ruto is a Kalenjin and Kenyatta is a Kikuyu, Kenya's largest ethnic group.
The ICC's decision, which the court said was based on about 30,000 pages of evidence, could backfire on the leading presidential contender, Prime Minister Raila Odinga, who is accused by Kenyatta and Ruto supporters of trying to exploit the criminal charges for his own political gain.
Kenyan radio presenter Joshua arap Sang and the head of Kenya's civil service, Francis Muthaura, must also stand trial, the ICC judges ruled. Sang and Muthaura said they would use all legal means to contest the ICC decision.
Analysts said Kibaki's legacy was on the line.
Muthaura is one of his closest allies in office and his trial will at the very least tarnish the outgoing president's reputation and possibly even implicate Kibaki himself in the 2007-8 violence, said Mair.
The judges said there was insufficient evidence for action against Henry Kosgey, former industrialisation minister, and Mohammed Hussein Ali, who was police commissioner at the time of the violence.
Today's decisions move forward the search for justice for those who lost their lives and their homes in Kenya's 2007-2008 election violence, said Elizabeth Evenson, senior counsel at the International Justice Program of Human Rights Watch.
Some victims welcomed the ruling. Others said the courts should really be trying the people who carried out the attacks that drove hundreds of thousands from their homes.
Justice has been done, said Harrisson Macharia, whose daughter was killed when machete-wielding Ruto supporters herded women and children into a church and set it ablaze on New Year's Day 2008.
We want justice for everyone who was victimised by the violence. We victims are now satisfied with the outcome. We suffered a lot, and we are now waiting for the trial.
Outside where the church used to be in Kiambaa, near the Rift Valley town of Eldoret, there are 36 weathered, wooden crosses standing just under knee height, some disappearing under long grass. Twenty four of them say only RIP Unknown.
In a camp for the displaced near Naivasha, Samuel Mathenge said those who carried out the attacks were known to them, and they should be the ones to face trial.
The decision by the ICC has surprised us as the guilty have been let go scot-free while the innocent have a case to answer, said Mathenge, a Kikuyu.
The only thing left is to pray for Uhuru as he came to our aid at the height of the violence and hope at the end he will be found innocent.
(Additional reporting by Gilbert Kreijger in Amsterdam, Jacob Kuehn in Eldoret, Beatrice Gachenge in Gatundu, George Obulutsa, Katy Migiro, Kevin Mwanza, Wangui Kanina, Yara Bayoumy and Duncan Miriri in Nairobi; Editing by David Clarke and Robert Woodward)