Chadians voted on Monday in a referendum on a constitutional amendment that would allow President Idriss Deby to run for a third term.
But opposition parties, along with rights activists, have said the result was fixed in advance and have called for a boycott of the vote. Provisional results are due on 20 June.
The constitutional amendment, already approved by parliament, lifts a two-term limit for a head of state and an age limit of 70 for candidates for the post.
Deby came to power in a 1990 coup but was elected to presidential office in 1996 in the country's first multi-party presidential vote since independence from France in 1960. He was re-elected in 2001 and survived a coup attempt last year.
As polling opened across the arid land-locked country at 7 a.m. small groups of voters turned out but there were no long queues visible in the capital N'Djamena.
"We have come to renew our confidence in President Deby who has been working for our country for 15 years," one voter, who asked to be identified simply as Abakar, told IRIN. "You will say this is not a presidential vote, but we want to express our confidence in him and ask him to run in 2006."
But an opposition supporter, who declined his identity, said: "Why vote? Others will vote for me. I've seen few people turn out so far, yet at the end of the day they'll say it was a massive 'Yes' vote."
The Coordination of Parties in Defence of the Constitution (CPDC), a loose grouping of about 30 parties, had called for a boycott and CPDC spokesman Ibni Saleh told IRIN on Monday "the boycott is massive."
In the referendum, voters are also being asked to rubberstamp proposals to abolish the Senate and replace it with an Economic, Social and Cultural Council, whose members would all be nominated by the president.
But it is the removal of the limit on presidential terms that has upset many.
"The revision of the constitution aims to put a dynasty in place in Chad, to maintain Deby and his clan in power," the CPDC's Saleh said. "That's why we are calling on Chadians to stay at home, to have a day of national mourning, to show the international community that this referendum is not democratic."
The government says the constitution needs to be updated to correct its shortcomings. It points out that amendments have already been made on two occasions, once in 1989 and again in 1996.
"Our constitution has shown itself to be lacking certain things and we must adapt to the realities we now face," Mahamat Barada, an activist of the ruling Patriotic Salvation Movement (MPS), told IRIN last week.
The move has the overwhelming support of parliament, where Deby's MPS party holds 113 of the 155 seats. Just over a year ago, parliament approved the latest batch of constitutional amendments almost unanimously and now the changes must be ratified by referendum to become law.
Meanwhile Deby himself was quoted by Reuters news agency as saying that the vote was not designed to keep him in office.
"We are not voting for one person," he said, "Those who have called for a boycott will see the results for themselves."
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