Government officials have set up centers to monitor hate speech and any tribal incitement, which was a major cause of violence in the last election.

It has also dispatched thousands of police officers nationwide to boost security, especially in areas most affected in the 2007 election.

In a video message, U.S. President Barack Obama urged Kenyans to put aside tribal rivalries and maintain peace.

"Kenyans have made remarkable progress since the devastating violence that followed the elections five years ago," he said. "Lives and communities have been rebuilt, the economy has rebounded, and Kenyans have peacefully stood together to pass a historic constitution and advance important political reforms.

Though an uneasy calm has returned, Kenyans remain wary.

Some are moving to areas dominated by their own ethnic groups to ensure strength in numbers during the election season.

Though there have been widespread peace efforts, rights groups remain concerned that the underlying issue of tribal rivalry still exists.

Who are the main presidential candidates?

Of the total eight candidates, there are two main front-runners: Uhuru Kenyatta and Raila Odinga, the sons of the nation's first president and vice president, respectively.

Though Kenyatta and Odinga are the main players, there are eight presidential contenders, including veteran politician Martha Karua, who, if she won, would be the first female president in Kenya.

The five others are Musalia Mudavadi, Peter Kenneth, Mohammed Dida, Paul Muite and James Ole Kiyiapi.

What other elections are being held?

In addition to the presidential poll, voters will also pick senators, governors, members of parliament and county representatives -- all under the new constitution.

The nation of about 40 million has a little more than 14 million registered voters.

What happens after the poll?

The winner must get more than 50% of the total votes to avoid a runoff in April. If there is no second round of voting, a new president will be sworn in later this month.