N'DJAMENA, Chad -- Chadian President Idriss Deby Itno was widely expected to win re-election Sunday, extending his three-decade-long rule in this central African nation after three top opposition politicians chose not to take part in the vote.

Some polling stations opened later than expected in the capital of N'Djamena, where streets are lined with giant portraits of Deby. A constitutional referendum approved several years ago means he can now serve two more terms, potentially stay in power until 2033.

"I invite all Chadians to go to the polls to choose the candidate of their choice," the longtime president said after casting his ballot Sunday alongside his wife. "I have seen since yesterday that the boycott order has not been respected, so people must go out to perform their civic duties."

While running for a sixth time in this oil-producing country, Deby has campaigned on promises of building schools, paving roads and improving living conditions in this country that remains one of the least developed in the world.

The top opposition candidate remaining in the race is Albert Pahimi Padacke, a one-time Deby ally who served as prime minister from 2016 to 2018. Five other lesser known candidates are also taking part, including one that has publicly accused Deby's party -- the Patriotic Salvation Movement, or MPS, of using state resources to campaign.

Human rights groups also say Chadian authorities have stepped up restrictions in the lead-up to the vote, including long internet shutdowns and arbitrary arrests.

"I decided not to vote this year. We already know the results, because the MPS will win and there is no point in wasting time," said Jean Luc Madjilem, a N'Djamena resident.

Others, though, said they were turning out despite the opposition concerns about the fairness of the vote.

"It is useless to respect the boycott order. We have to fulfil this duty and we will see what the result of this presidential election will be," said Ali Mahamat , a voter in the 2nd district of the capital.

Deby, a former army commander-in-chief, first came to power in 1990 when his rebel forces overthrew then-President Hissene Habre, who was later convicted of human rights abuses at an international tribunal in Senegal. Deby has continued to win re-election over the years, last drawing 61.5% of the vote in the last 2016 election.

The landlocked nation of Chad is home to nearly half a million refugees from neighbouring Sudan, Nigeria and Central African Republic. Another 330,000 Chadians are internally displaced, the majority in the volatile Lake Chad region where Boko Haram militants are active.

The military base for France's Operation Barkhane in the Sahel region is also based in Chad, a French colony until 1960. And the country has been a major contributor the U.N. peacekeeping mission in Mali, accounting for many of the casualties because of attacks by Islamic extremists.