Idriss Deby, a former army chief who seized control of the former French colony in 1990 and won elections in 1996 and 2001, is seen easily winning a third five-year term in Wednesday's poll.
He faces no serious challenge from four other contenders who are mostly government allies.
His supporters portray him as a guarantor of stability against the spillover of conflict from the violence-torn Darfur region of neighbouring Sudan, whose government Deby accuses of trying to overthrow him.
But the president's opponents say his nearly 16-year rule has become increasingly corrupt and dictatorial in the landlocked central African state.
They say he has favoured members of his own Zaghawa clan over other groups in the ethnically diverse country. Several high-ranking kinsmen have deserted in recent months to join the rebels.
Two anti-Deby groups, the United Front for Democratic Change (FUC) which attacked N'Djamena nearly three weeks ago, and the Rally of Democratic Forces (RAFD), said recently they would co-ordinate military operations against the elections.
Fears of attack
Deby's army is on alert to thwart any threat to derail voting by the fighters who, on April 13, rolled in from the eastern border with Sudan in pickups to raid the capital N'Djamena.
Fearing a fresh attack, some residents of the dusty, dilapidated capital have taken refuge across the river in nearby Cameroon.
The April 13 assault was defeated by government troops helped by intelligence passed on by a 1,200-strong French military contingent stationed in Chad.
Soldiers foiled an April 13 rebel
attack on N'Djamena, the capital
French Mirage jets, which fired a warning shot at one rebel column, fly daily reconnaissance missions over the vast, arid country.
"This is an election being held under French protection - have you heard the jets? It's a complete farce," said Ngarledjy Yorongar, an opponent of Deby, whose Action for the Republic Federation is joining 15 other opposition parties in a boycott of the election.
Government ministers call the boycott "anti-democratic" and say the armed groups' attempts to seize power by force are a recipe for further chaos and bloodshed in a country plagued by ethnic conflict and civil war since its independence in 1960.
Ahmat Allam-Mi, the foreign minister, said: "It's them [the opposition] who will be electing Deby by refusing to take part in the elections."
In addition to the security threat, the ruggedness of the country - which includes mountains and deserts and dry, brush-covered savannah - makes holding an election a logistical challenge.
Many of the 5.8 million voters will have to walk several miles to cast their ballots and officials say official results may take several days to be released.
Although Chad became an oil producer in 2003, it remains one of Africa's poorest and most corrupt countries.
Most of its 10 million inhabitants live in squalid towns and villages with scarce water and power, and only rudimentary health and education facilities.