The tanker tipped over and spilled fuel onto cars and into shops before it ignited, witnesses said.
'Run for my life'
Matthew Eliagwu, a 61-year-old shop owner, said: "I heard a loud bang and I came out of my shop. I saw fire running down towards my shop, but I escaped.
"It was impossible to help anyone, because I had to run for my life."
An AFP reporter in Port Harcourt said the tanker was carrying stolen petrol and the driver was trying to escape a team of policemen chasing the vehicle.
Police could not immediately confirm the number of casualties.
The accident comes a day after armed groups fighting for autonomy blew up an oil tanker in the region, wounding three people.