At least 20 people were killed and dozens more injured after two trains carrying hundreds of passengers collided in the Landhi area of Pakistan's southern port city of Karachi on Thursday, officials said.
Witnesses described watching in horror as the incoming Zakaria Express train from the central city of Multan sped into Karachi's Quaidabad Railway Station and rammed into the Fareed Express train, which was stationary.
They added that the roar of the crash was swiftly followed by the screams of people trapped inside.
Officials said rescuers arrived at the scene of the accident and began using metal-cutting equipment and heavy cranes to pull all the passengers from the twisted wreckage.
Many victims were rushed to Karachi's Jinnah Hospital, where the injured lay screaming and crying while medics rushed to help them.
Some victims appeared too stunned to talk. Many had head and foot injuries, and at least one man had his leg amputated below the knee.
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.
He said an inquiry has been launched into the cause of the accident.
"The death toll has now risen to 20, the condition of several more is critical," said Seemi Jamali, a spokeswoman at Jinnah Hospital. And up to 60 people had been injured in the crash according to Nazeer.
Pakistan's Federal Minister for Railways Khawaja Saad Rafique pointed at negligence as a possible cause, with a stop signal being ignored. The whereabouts of the driver and assistant driver of the Zakaria Express were unknown, he said.
Rafique ordered an investigation which would be completed within 72 hours. The minister said that those found responsible will not be able to "escape legal action and punishment."
Rail accidents are common in Pakistan, which inherited thousands of kilometres of track and trains from former colonial power Britain. The railways have seen decades of decline due to corruption, mismanagement and lack of investment.
Thursday's crash was the second this year involving the Fareed Express. In February, the northbound train hit a van at a crossing in southern Pakistan, killing eight people from the same family.
In September, four people were killed and more than 100 injured when two trains collided near the central city of Multan.