The crash happened at about 5:30 pm (1630 GMT) on Thursday on the Lagos-Ibadan expressway, one of the main routes out of the city. Fifty-four vehicles were gutted.
Commuters were forced to walk as traffic backed up throughout the city and as a clean-up operation continued through the night until the road was reopened at about 7:30 am.
At first light, the burnt-out shells of cars and buses were still being towed from the highway, which was blackened and covered with ash for more than a kilometre (half a miles)
Petrol tanker fires are not uncommon on Nigeria's roads, which are often in disrepair and where vehicles are frequently old and badly maintained.
Blocks are often put behind the wheels of ageing, rusting trucks when they are stationary in traffic.
The authorities said the initial crash was caused by brake failure. Motorists caught up in the chaos also questioned why fully-laden tankers were allowed on the road at peak periods.
The stricken tanker in Thursday's crash was carrying 33,000 litres (8,700 US gallons) of petrol at the time.
Suleiman Yakubu, from the National Emergency Management Agency (NEMA), said the location - Otedola Bridge in the Berger area - was a downhill stretch which drivers often take at speed.
Lagos state police spokesman Chike Oti was quoted in local media as saying drivers of tankers and articulated lorries had been warned to "put in all necessary safety checks".
Another road safety official, who asked not to be identified, added: "The blame for this accident should be put on the driver and the owner of the vehicle. "People don't keep their vehicles in good condition."
Bolarinwa Dolarinwa Mufutau, a roadside mechanic, said he had just finished repairing a car when he saw the tanker rolling back on the road.
"I could also hear the driver shouting 'brake failure, brake failure'. I saw the motor boy (assistant) trying to put a wedge to stop the tanker from rolling back," he told AFP.
"But the truck ran over the wedge backwards and immediately there was a bang. I thought it was a bomb and there was an explosion and fire everywhere, so I started running away."
Fire erupted within seconds, forcing motorists to abandon their cars and flee, he added.
