The lorry was parked at a stop on the outskirts of Hyderabad, 160km north of Karachi, Pakistan's commercial capital.
The blast destroyed around a dozen vehicles and at least 10 shops.
Nawaz Abbasi, a medical superintendent at the city's Liaqat University Hospital, said his hospital had received 14 bodies, and at least 30 people wounded.
Fayyaz Leghari, a senior police officer, told the Reuters news agency no evidence suggesting it was a bomb or a suicide attack was found.
Pakistan has been hit hard by sporadic violence in recent months as it fights Taliban and al-Qaeda fighters in the northwest.
In the latest clash of its kind, security forces in the northwestern Orakzai region killed 12 fighters after their checkpoint came under attack, a government official said.
Two soldiers were wounded in the gun battle.
No independent verification of official figures of casualties was available.