|Chadian workers guide a pipe down a well in the Doba oil fields in southern Chad where a $3.7 billion project was officially inaugurated on Oct. 10, 2003|
Diplomats, opposition leaders and human rights activists say registration for the vote was fixed. The government denies this.
Chad's National Electoral Commission says 5.6 million voters have registered for the ballot, out of a total population of about nine million, more than half of whom are under 18.
The opposition has called for a boycott.
A "yes" vote would give former guerrilla and 1990 coup leader Idriss Deby a chance to run for a third term in 2006.
Voters will also decide whether to abolish the Senate and replace it with a special council appointed by the president. Oil researcher and close Chad watcher Ian Gary from the pro-transparency group, Catholic Relief Services, says the Senate never existed anyway.
"The Senate was something that was called for in the constitution, but it's never actually been constituted," he said. "So, they are actually abolishing something that was never put into place. And I'm not clear how important that body would have been, had it been constituted. But in the end, only the national assembly is functioning as a representative body in Chad."
Parliament gave preliminary approval to the constitutional changes last year, as opposition parties boycotted that vote, as well. The opposition is refusing to take part in a process, which it says is turning Chad into a monarchy.
There is also a clause, which would make constitutional revisions a presidential prerogative.
In the polling booths, voters will choose a white paper to vote "yes" for all the changes and a black one to vote "no."
The vote comes amid growing fears of instability. Mr. Deby's rule has been disrupted by a brief military mutiny last year, his own suspected poor health, and the spillover of Sudan's Darfur war, with 200,000 refugees fleeing into Chad.
Some of the populations persecuted in Darfur are from the same Zaghawa ethnic group as the 53-year-old Mr. Deby.
Mr. Gary, from the group, Catholic Relief Services, points out the vote also comes amid growing oil revenue.
"[In] 2004, the government received approximately $150 million from oil revenues. In 2005, we'll look in the neighborhood of $250 or $260 million, depending on the oil price, as well as depending on the outcome of an ongoing dispute between the government and the oil consortium over the calculation of revenue payment and the discount that's subtracted out of the oil price for the low quality of the oil that's coming from southern Chad through the pipeline," explained Mr. Gary.
The World Bank helped pay for the pipeline, which goes from Chad's southern oilfields through Cameroon to the Atlantic Ocean. An oil revenue management law was also passed as part of the deal to ensure help for about 80 percent of the population, which lives on less than $1 a day.