Kolelas “died in the medical aircraft which came to get him from Brazzaville on Sunday afternoon,” his campaign manager Christian Cyr Rodrigue Mayanda told the AFP news agency.
The 61-year-old tested positive for COVID-19 on Friday afternoon, and could not host his last campaign meeting in Brazzaville.
The presidential election had been boycotted by the main opposition and took place under an internet blackout.
Mayanda called on Kolelas’ supporters to rally at 11 a.m. local time on Monday. “We’ll continue to count the ballots,” he said. “He was ahead in a number of areas.”
Critics had voiced concerns about the transparency of polls, which were seen as tilted in favor of veteran leader Denis Sassou Nguesso.
Kolelas was the main rival to Sassou Nguesso, who has been the Republic of Congo’s leader for total of 36 years since 1979.
On Saturday, the challenger had posted a video after being taken ill, declaring he was “battling against death.”
“Rise up as one person... I’m fighting on my deathbed, you too fight for your change,” he urged his supporters. Kolelas added that the election was “about the future of your children”.
Although provisional election results are not expected for days, Sassou Nguesso is widely predicted to be the winner.