Tension has been high ahead of the second round run-off vote, with several people killed in clashes between supporters of the rival contenders during campaigning.
Voters are to choose between the incumbent President Joseph Kabila and his fierce rival, the current Vice President Jean-Pierre Bemba.
Kabila is favourite to win, after gaining 45 percent of the vote in the first round in July, against Bemba's 20 percent. Violence followed news of these results, with 23 people killed in fighting between armed supporters of the two men.
This time, the president is expected to be boosted by support from backers of another candidate who dropped out after the first round.
Kabila, at 35 the youngest head of state in Africa, took the helm of this nation the size of western Europe when the former leader, his father Laurent Desire Kabila, was assassinated in 2001. His traditional power base is in the east of the country.
Bemba, 43, formerly led a rebel group that fought to overthrow the elder Kabila in a civil war that shook the country from 1998 to 2003. He is now a rich businessman with interests in the mobile telephone, air freight and television sectors. He enjoys strong support in the capital Kinshasa.
Voters will also cast ballots for local officials, voting for their choice of provincial deputies who will be responsible later for electing provincial governors and senators.
The parties headed by Kabila and Bemba called on the country to vote "in a calm, orderly and peaceful manner", in a joint statement on Saturday.
The 50,045 polling stations will open at 6:00 am (0500 GMT) in the western time zone and at 4:00 am (0300 GMT) in the east. They will close at 5:00 pm in both zones and counting will start immediately.
Provisional results are expected on November 19.
Fearing an increasing risk of violence when the vote-counting starts, embassies in Kinshasa gave strict advice to their nationals to limit their movements.
"There was an escalation towards the end of the campaigns. Everyone was very agitated," a diplomat in Kinshasa told AFP, judging that the polling would "go well", but that the tension could "increase afterwards".
The last day before the decisive polls passed calmly on Saturday, however.
More than 1,000 international and 40,000 Congolese observers will monitor the polls, while some 80,000 policemen, 17,600 United Nations troops and 1,200 soldiers from the European Union will maintain security.
"Our great and beautiful country which has suffered for too long ... deserves to take once and for all the road of peace and democracy," said a statement signed by senior representatives of the Kabila and Bemba camps.

Copyright AFP (Agence France-Presse), 2006