The tanker swerved as it was trying to avoid a collision with three oncoming vehicles including a bus, said Kayode Olagunju, sector commander of the Federal Road Safety Commission in the southern Rivers state.
Residents said that shortly after the collision hundreds of locals flocked to the site to collect the spilling fuel.
"Then there was an explosion followed by fire," Mr Olagunju told AFP. "Ninety-three were burned to death on the spot. Two died later in the hospital (and) 18 people were seriously injured."
In a statement, the National Emergency Management Agency (NEMA) gave the same figures.
An AFP photographer at the scene said many of those killed were motorcycle taxi operators, known locally as "Okada", who raced to fill up their tanks after learning of the crash.
Mr Olagunju said at least 34 motorcycles were destroyed in the blaze.
The accident happened in an area called Ahoada near the oil hub of Port Harcourt in Nigeria's crude-producing Niger Delta region.
Motorcycle taxi driver Kingsley Jafure said the vehicle collision occurred at roughly 6:00 am, and the spilled petrol caught fire about 90 minutes later, but that time sequence could not be immediately confirmed by officials.
"At about 7:30 while I was inside trying to decide whether to go (scoop fuel) or not. That is when I saw that the tanker exploded," Jafure said.
The area had been cordoned off by security forces and a large number of rescue officials were on the ground, said an AFP correspondent.
The NEMA statement said "rescue workers from the police, road safety, fire service, civil defence and NEMA were at the scene to evacuate victims and control the traffic."
Major accidents, often involving large-haul trucks, are common in Nigeria, where many of the roads in terrible condition.
Lorries operating on the country's road are often old and poorly maintained and road worthiness checks are scant.
Abandoned trucks, some of them destroyed by heavy collisions, can regularly be seen along major Nigerian motorways.
In March, a petrol tanker caught fire after skidding off the road in southern Port Harcourt, killing six people and injuring several others.
While in April last year, a fuel tanker overturned at an army checkpoint in central Nigeria, sparking an inferno in which some 50 people were killed.