Rescuers armed with metal-cutting equipment and heavy cranes managed to pull all the passengers from the twisted wreckage, officials said.

Many were rushed to Karachi's Jinnah Hospital, where an AFP reporter described horrifying scenes as the injured lay screaming and crying while medics rushed to help them. 

"I am dying, I am dying, please, please, I am dying," cried Abdul Ghaffar, 55, as doctors tried to move his legs and hands. He appeared to have multiple injuries, while his children and wife were also wounded and lay on beds nearby.

Other victims appeared too stunned to talk. Many had head and foot injuries, and at least one man had his leg amputated below the knee. 

Dr Kaleem Shaikh, the senior medical officer at Jinnah Hospital, put the death toll at 21 with 65 wounded. 

Among the dead, he said, were a couple and their twin daughters.

Casualties were still being counted but there could have been a total of up to 1,000 passengers on board the trains when the accident occurred, said Nasir Nazeer, an administrative official in Karachi.

Pakistan's railways minister claimed the driver had been asleep, but local officials said only that an inquiry has been opened into the cause of the accident.