Firefighters from the CPD were deployed on the scene only to find that the smoke was being caused by the electrical generator at the premises.
The incident happened on Tuesday morning, at around 7 am.
More than 70 firefighters were battling the blaze, assisted by five helicopters and two planes, with volunteers also helping.
The man, a Rwandan refugee, worked as a volunteer for the cathedral and had been in charge of locking up the building on Friday night, LCI said, citing prosecutor Pierre Sennes.
Health experts also fear the smoke that often blankets the region during the dry season, causing respiratory problems, could cause complications for COVID-19 patients.
Australian authorities warned on Friday of severe fire danger in densely populated areas this weekend, declaring a state of emergency in the capital, Canberra, as soaring temperatures and strong whipped up huge, unpredictable blazes.