A lorry carrying fuel burst into flames near the Pakistani city of Ahmedpur East, killing at least 140 people, local officials say.
Villagers had gathered, reportedly to collect fuel leaking from the crashed tanker, when it caught fire. Dozens are being treated in hospital.
It appears the tanker blew a tyre while rounding a sharp bend in the road.
The fire was sparked by a passer-by lighting a cigarette, a rescue services spokesman told the BBC.
“The incident, which was a minor [one], turned into a major blast,” Jam Sajjad said.
Prime Minister Nawaz Sharif is cutting short a visit to London in response to the incident, the Pakistani government news agency, APP, reported.
Army helicopters were dispatched to ferry casualties to hospitals, army spokesman Maj Gen Asif Ghafoor said in a tweet. There are fears that the death toll could rise further.
Some of the victims may only be identified by DNA sampling, as the bodies were so badly burned in the incident, reports say.
Police sources told APP that the tanker had been transporting 25,000 litres (5,500 gallons) of fuel from Karachi to Lahore.
It left the road and overturned in the Kachi Pul area, they said, about 8km (five miles) from Ahmedpur East, in Punjab’s Bahawalpur district.
Police sources said that people in nearby villages had rushed to the scene to collect fuel in pots, and also made phone calls to their relatives in other villages to come to collect the fuel.
Traffic police had tried in vain to keep the crowds away from the tanker as local people, including women and children, continued collecting fuel, the sources said.
A huge fire then erupted, engulfing all the people standing around the vehicle, they said.
Emergency services have struggled to find burns units to treat the injured, APP says.
The Ahmedpur Sharqia Health Centre and Bahawal Victoria Hospital have no such units, but some casualties were taken to Nishtar hospital in Multan and others to hospitals in the city of Bahawalpur, about 50km away.
The country’s roads are notoriously dangerous, with poor vehicle maintenance and reckless driving contributing to the problem.