Kenya’s Deputy President William Ruto formed an army prior to the elections in 2007 “to go to war for him”, the prosecution has alleged at his trial.
He pleaded not guilty to crimes against humanity charges as the trial began at the International Criminal Court (ICC).
Mr Ruto and President Uhuru Kenyatta are accused of orchestrating violence after elections in 2007, and are being tried separately at The Hague.
Mr Ruto becomes the first serving official to appear at the ICC.
The two trials are seen as a crucial test of the ICC’s ability to prosecute political leaders.
This is a politically controversial trial with a complex legal history, says the BBC’s Anna Holligan in The Hague.
Mr Kenyatta and Mr Ruto were on opposite sides during the 2007 election and are accused of orchestrating attacks on members of each other’s ethnic groups.
They formed an alliance for elections in March, saying they were an example of reconciliation.
Analysts say the ICC prosecutions bolstered their campaign as they portrayed it as foreign interference in Kenya’s domestic affairs.
Mr Ruto watched and smiled during proceedings and pleaded not guilty to each of the three counts of murder, persecution and forcible transfer of people, our correspondent says.
Mr Ruto’s defence lawyer, Karim Khan, accused the prosecution of building its case on “a conspiracy of lies”.
“We say that there is a rotten underbelly of this case that the prosecutor has swallowed hook, line and sinker, indifferent to the truth, all too eager to latch on to any… story that somehow ticks the boxes that we have to tick [to support charges],” Mr Khan said.
He downplayed claims his client was driven by ethnic hatred, telling the judges that two of Mr Ruto’s sisters of the Kalenjin ethnic group were married to members of the rival Kikuyu group.
Chief prosecutor Fatou Bensouda said Mr Ruto had planned violence over an 18-month period prior to the 2007 elections, exploiting existing tensions between his Kalenjin group and Mr Kenyatta’s Kikuyu group.
Mr Ruto used his power to procure weapons, secure funds and co-ordinate the violence, Ms Bensouda said.
A group of Kenyan MPs and other supporters welcomed Mr Ruto and his co-accused Joshua arap Sang as they arrived for the trial, AFP reports.
He is the head of a Kalenjin-language radio station and is accused of whipping up ethnic hatred.
In Kenya, many people are following the case closely and opinion is split with opposition supporters welcoming the trial and government supporters opposed to it, says the BBC’s Caroline Karobia in the capital, Nairobi.
Some 1,200 people were killed and 600,000 forced from their homes in weeks of violence after the disputed December 2007 election.
More than 40,000 people are estimated to be still living in camps, which Mr Kenyatta last week promised to close by 20 September.
On Sunday, he gave cheques worth more than $4,500 (£3,000) per family so they could move out of camps and rebuild their lives.
Ex-UN chief Kofi Annan said, in an article in The New York Times, that the trials were not an assault on Kenya’s sovereignty but the “first steps toward a sustainable peace that Kenyans want, deeply”.
“Making clear that no one is above the law is essential to combat decades of the use of violence for political ends by Kenya’s political elite,” he wrote.
Mr Annan brokered the peace deal that brought an end to the brutal killings.
It included an agreement that those responsible for the violence must be held to account.
A commission was set up to investigate the violence and it recommended that if efforts to establish special tribunals in Kenya failed, the matter should be sent to The Hague.
Kenya repeatedly failed to set up such tribunals and so the ICC indicted those it said bore the greatest responsibility for the violence.
The ICC on Monday said the two trials would not clash, after Mr Kenyatta warned that the constitution prevented the two men from being abroad at the same time.
The president is due to go on trial in November. He also denies charges of fuelling violence.
The judges said the two cases could be heard alternately – in blocks of four weeks.
On Thursday, Kenya’s parliament passed a motion calling for the country to withdraw from the ICC.
The court said the cases would continue, even if Kenya withdrew.
In May, the African Union (AU) accused the ICC of “hunting” Africans because of their race and urged it to drop the Kenyan cases.
The ICC says it pursues justice impartially and will not allow perpetrators of violence to go unpunished.
The court was set up in 2002 to deal with genocide, crimes against humanity, war crimes and the crime of aggression.
It has been ratified by 122 countries, including 34 in Africa.