Up to 11.6 million eligible voters are expected to approve or reject the draft constitution, which has split President Mwai Kibaki's government and sparked hot-tempered campaigns across the east African nation.
Some 150 observers, including those from the European Union, are monitoring the voting that opened at 7am (0400 GMT) and closes at 5pm on Monday.
At least eight people have been killed in campaign-related violence in the recent weeks.
In July, riots convulsed the Kenyan capital when parliament revised the draft, leaving one person dead.
Hundreds queued in the western city of Kisumu, where four people were killed last month, amid a massive police presence.