The incident occurred when the Zakaria Express train from Multan collided with Fareed Express from Lahore, according to Ashraf Lanjaar, director general of operations at the Ministry of Railways.
Both trains run every 15 minutes from their respective stations during morning rush hour. The Zakaria train has a capacity of 1,232 people and the Fareed train 1,320.
The Fareed Express was already standing at Landhi Railway station when Zakaria Express crashed into it. It appears that the driver of Zakaria express ignored the signals on the line, according to Lanjaar. The station is on the Karachi city main line.
Video from the scene shows extensive damage to at least one train carriage, with rescue and relief and other officials on the scene.
Seemi Jamali, head of ER at Jinnah Hospital, tells CNN she and her team have been treating the injured and receiving the bodies of those killed since the accident occurred.
Railway Minister Khawaja Saad Rafique tweeted his condolences.
"I am directly in contact with (Railway district superintendent), supervising the emergency activities," he said in the post. "Really sad over loss of precious souls."
A followup tweet promises an independent inquiry into the accident.
In a statement released by his office, Pakistani Prime Minister Muhammad Nawaz Sharif expressed his grief and sorrow over the loss of life, and directed an immediate rescue and relief operation, as there may be injured and/or stranded individuals at the scene.
Sharif also asked concerned authorities to investigate the cause of the accident, according to the statement, adding, "those found responsible for this loss must be punished."
In September, two trains collided
around 25 km outside the city of Multan in Punjab province, leaving six dead and scores injured.