At least 17 people have been killed and dozens more injured after two trains carrying hundreds of passengers collided in Pakistan's southern port city, according to officials.
The accident occurred when the incoming Zakria Express from the central city of Multan collided with the Fareed Express, from Lahore, early on Thursday, as it waited at Quaidabad Station, also known as Jumma Goth, in the Landi neighbourhood of Karachi.
A senior administration official in Karachi said there could have been up to 1,000 passengers in total on board the trains when the accident occurred.
"At the moment, we don't know the exact number of casualties but there could be up to 1,000 passengers in both trains," Nasir Nazeer, district coordination officer in Karachi, told AFP news agency.
"We have sent heavy machinery and cutting equipment to remove the overturned cars from the track and rescue the trapped passengers inside the coaches."
Train services from Karachi to the rest of Pakistan have been suspended.
Train accidents are common in Pakistan, which inherited thousands of kilometres of track and trains from former its colonial power, Great Britain.
Thursday's crash was the second this year involving the Fareed Express.
In February the north-bound 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.