Kenyans are voting in a referendum on a new constitution, the centrepiece of measures designed to reform politics.
It limits the powers of the president and sets up a commission to settle land disputes that fuelled past violence.
The referendum was part of a deal that ended deadly clashes after a disputed election in December 2007 when more than 1,000 people were killed.
Both the president and the prime minister are backing the "Yes" vote.
And opinion polls suggest the new constitution will be approved.
President Mwai Kibaki has appealed to Kenyans to vote peacefully and in large numbers.
"Security has been stepped up in all parts of the country," Mr Kibaki said.
In a televised address, he said the referendum marked a "defining moment" in the country's history.
He and his rival in the 2007 election, Prime Minister Raila Odinga, have both backed the "Yes" campaign.
Polling stations opened at 0600 (0300 GMT), with long queues already waiting in some places. Some people told the BBC they had been waiting several hours for voting to begin.
The current constitution was negotiated with the British in London in the early 1960s.
"Let us also embrace one another as brothers and sisters even after the referendum," Mr Kibaki said.
The post-election violence in 2007 and 2008 left about 1,300 people dead and 300,000 homeless.
Land tension
There has been some violence during the referendum campaign - most notably in June, when a grenade attack on a Church-organised "No" rally in the capital, Nairobi, left six people dead.
Correspondents say tension is especially high in the volatile Rift Valley Province, home to most of the "No" campaigners, where land is a contentious issue.
Disputes between rival ethnic groups over land were behind some of the most violent incidents after the December 2007 election.
Those in favour of the new constitution argue that for the first time it introduces a sensible approach to land reform by stating that land acquired illegally can be repossessed.
But those against it say this will increase ethnic divisions and trigger chaos.
Ahead of the vote, UK-based human rights group Amnesty International has urged politicians not to stir up ethnic hatred.
"Another bloodbath is not inevitable so long as Kenyan politicians act responsibly, do not stoke ethnic tensions," Justus Nyang'aya, Amnesty International's Kenya director, said.
"The referendum also provides an opportunity for Kenya's security forces to show that they are capable of carrying out their professional duties in line with international human rights standards."
