Guy Brice Parfait Kolelas, 61, had been seen in a video shared on social media hours before the poll, receiving oxygen treatment at an undisclosed medical facility.
He urged his supporters to head to the polls and “vote for change. I will not have fought for nothing. Fight for it,” he said.
Kolelas, a former government minister and one of the main contenders in Sunday’s poll, had hoped to topple the reign of 77-year-old President Denis Sassou Nguesso, who has led the country for decades.
Kolelas was a runner-up to Nguesso in the 2016 presidential election, where he amassed about 15% of the total ballot.
President Nguesso, who is looking to extend his 36-year reign is expected to cruise to victory ahead of Kolelas and five other presidential contenders as vote counting continues in the oil-rich Central African country.
More than half of the country’s five million people registered to vote in the poll, officials said.
He reclaimed power in 1997 and has led the country ever since.