Thirty others were injured in the incident, which happened outside a hospital in the centre of the city late on Sunday.
There was no official statement indicating it was an attack.
The explosion happened when a car which had been travelling against the traffic hit three others, officials said.
It sparked a fire that forced the evacuation of nearby buildings. Images of blazing cars and plumes of smoke have been widely shared on social media.
Dozens of patients from the nearby National Cancer Institute were evacuated to other hospitals, the health ministry said.
It added that several people caught up in the blast were in a critical condition.
The explosion caused damage to the structure of the hospital, one witness told Reuters news agency.
Images showed investigators scouring the scene overnight while armed police stood guard.
The country’s public prosecutor has opened an investigation.