At least 20 people have been killed and dozens more injured after two trains carrying hundreds of passengers collided in Pakistan's port city of Karachi during morning rush hour on Thursday, officials said.

Witnesses described watching in horror as one train sped into the city's Quaidabad Railway Station and rammed into the second, which was stationary, with the roar of the crash swiftly followed by the screams of people trapped inside.

Officials said rescuers armed with metal-cutting equipment and heavy cranes had managed to pull all the passengers from the twisted wreckage.