The 77-year-old is widely expected to win against six contenders, led by economist and 2016 presidential runner-up Guy-Brice Parfait Kolelas, who revealed Saturday that he is gravely ill.

Sassou Nguesso, a former paratrooper, first rose to power in 1979 and has since accumulated 36 years in office, making him one of the world’s longest-serving leaders.

He is hoping for a first-round victory to secure a fourth term running the central African oil exporter.

Meanwhile, challenger Kolelas posted a video message online showing himself in a sickbed, declaring he was “battling against death” after taking off a respiratory mask.

“Rise up as one person… I’m fighting on my deathbed, you too fight for your change,” he urged supporters, saying the election was “about the future of your children” before replacing his mask.

Kolelas’ campaign did not say what illness had struck the 60-year-old, although his team said he could be transferred for treatment abroad even as voting takes place Sunday.

The runner-up in 2016’s presidential election, Kolelas is the only major candidate remaining to face Nguesso.