PM Raila Odinga and deputy PM Uhuru Kenyatta are favourites to succeed President Kibaki, in the first poll since the post-2007 election violence.
Thousands of supporters gathered for the two rallies in central Nairobi.
The president has urged Kenyans to vote peacefully on Monday and for the losers to accept defeat.
The election is the first to be held under a new constitution, adopted in 2010 in an attempt to avoid a repeat of ethnic clashes which led to more than 1,200 people being killed and an estimated 600,000 others being forced from their homes.
Uhuru Kenyatta addressed supporters at a rally at Uhuru park, around a mile north of Raila Odinga’s main campaign event at the Nyayo national stadium.
The BBC‘s Anne Soy, at the stadium, said the prime minister’s supporters came out in large numbers to hear the final speech of his campaign.
The third main candidate of the eight running for the presidency, former vice-president Wycliffe Musalia Mudavadi, was holding rallies in western Kenya, reports said.
Mwai Kabaki is standing down after two terms as president, but the events of 2007 continue to worry Kenyans.
The Kenya National Commission on Human Rights (KNCHR) this week released a report detailing cases of violence, hate speech and ethnic intimidation, warning that campaigns had “continued to raise emotions and inflame passions as they campaign for votes”.
Uhuru Kenyatta is facing trial at the International Criminal Court (ICC) in The Hague in a few weeks’ time on charges of orchestrating some of the bloodshed. He denies the charges.
After the 2007 poll, Raila Odinga said he had been cheated of victory by Mwai Kibaki’s supporters. Ensuing protests spiralled into nationwide inter-ethnic attacks.
Mr Odinga’s supporters say that if he loses this election, they will accept the result, our correspondent says.
His office denied a report in the Financial Times quoting him as saying that if he lost it would be “because of blackmail and intimidation”. In a statement, Mr Odinga said he felt “absolutely slandered”.
“Cast your vote and keep the peace,” President Kibaki said in a televised address to the nation on Friday.
“Let us send a clear message to the world that our democracy has come of age. A peaceful vote is a vote for a secure, prosperous and stable Kenya.”
The government says some 99,000 police officers are being deployed around the country, at polling stations and vote counts, in order to prevent a repeat of the violence.
Presidential candidates must secure support from across the country in order to be declared the victor, so they cannot just rely on support from their ethnic groups, as has been the case in previous elections.
If no candidate obtains more than 50% of the vote, there will be a run-off on 11 April.
In the previous election, Mr Odinga said he had been cheated of victory by allies of Mr Kibaki. Protests by his supporters turned violent and this spiralled into nationwide tit-for-tat attacks on members of ethnic groups seen as supporting one or other candidate.
This time there are eight presidential candidates.
Elections are also being held for members of parliament, new posts of regional governors and members of county assemblies.