Two trains collided early Thursday in the Pakistani port city of Karachi, killing at least 21 people.
An official at Karachi's Jinnah Hospital said as many as 50 others injured in the crash had been brought to the hospital.
A factory worker told the AFP news agency that an inbound train hit another train that was parked at the station.
"Suddenly another train came speeding in and smashed into the parked train," Ajab Gul said. "There were clouds of dust and smog. After that we heard screams. People inside the collided trains were screaming and crying."
News broadcasts showed rescuers cutting through wreckage as they tried to reach passengers trapped in train cars.
Wounded passengers on hospital beds described watching in horror as their Zakaria Express coming from the city of Multan rammed into another packed passenger train, which was parked at the station.
Authorities say an initial investigation into the accident suggests railway staff gave green signal to the train by mistake.
Speaking in Islamabad, Federal Minister for Railways Khawaja Saad Rafique said the drivers were first shown a yellow signal followed by a red signal.
He explained the two indications are a standard process to warn the driver to stop the train dead for a minute and then proceed very slowly. But the drivers ignored both signals, apparently causing the accident, the minister insisted.
According to local media, all train travel to and from Karachi has been suspended.
In September at least four people died and nearly 100 others were injured when a freight train collided with a passenger train near Multan.