Iran Press/ Africa: Fuel spilled before igniting, and the resulting inferno engulfed bystanders and vehicles at a busy junction.
Footage broadcast by local media outlets showed badly charred bodies in the streets surrounding the tanker.
After visiting the scene, Vice-President Mohamed Juldeh Jalloh said it was a "national disaster."
Some 100 people are being treated in hospitals and health centers around Freetown, some of whom are overwhelmed.
The explosion is believed to have happened at a junction outside the busy Choithram Supermarket in the densely populated Wellington suburb in the east of the city at around 22:00 GMT on Friday.
One report said a completely burnt bus while nearby shops and market stalls were caught up in flames.
Brima Bureh Sesay, Sierra Leone's National Disaster Management Agency head, said there were many "burnt corpses."
"It's a terrible, terrible accident," he told local media in a video shared from the scene.
At the scene of the accident, there are body parts still lying in burnt-out vehicles. Also, here are what's left of the petrol tanker and the truck that rammed into it.
Motorbike-taxi riders nearby started collecting the leaking fuel, causing a traffic jam. Shortly after, there was an explosion.
The fireballs spread into the surrounding area and onto the vehicles caught in the traffic.
The authorities say most of the dead have now been received at the central morgue in Freetown, with the critically injured admitted to hospitals across the city. The army, police, and fire force are all at the scene.
The port city of Freetown, which is home to just over a million people, has faced several severe disasters in recent years.
In March, more than 80 people were injured after a major fire in one of the city's slums left more than 5,000 people displaced.
And in 2017, over 1,000 people were killed after heavy rains led to a mudslide that swept through the city, leaving around 3,000 people homeless.
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