The accident happened at dawn near the town of Ghotki in Sindh province. The trains were packed with passengers.

Police said a train travelling to Karachi crashed into a broken-down train, catapulting carriages across the track into the path of another train.

The general manager of Pakistan Railways said the crash was caused by a train conductor misreading a signal.

People are crying, fathers are looking for children, husbands for their wives and brothers for their sisters." - Witness

Medical officials collating casualty figures from hospitals in the area said the death toll was expected to rise. A number of the injured are in a critical condition.

At least 13 train carriages were derailed. Shocked survivors described being thrown from their beds and seats.

"I woke up at the noise of a huge bang and then there was [a] big jerk and smoke all over the place," injured passenger Mohammad Amin told Reuters news agency.

"There was total darkness... I hit the floor and fainted."

Rescue workers have been cutting through twisted metal to reach the dead and injured trapped inside the wreckage. Bodies were strewn across the tracks. The army has cordoned off the crash site, and troops are helping with the rescue effort.

Train traffic along Pakistan's main north-south route has been badly disrupted, and a special relief train to bring survivors back to Karachi has been cancelled.

Worried families in Karachi have headed to the city's main train station, but officials said they had no passenger lists.

Many families are renting private cars in an effort to reach the crash site and look for loved ones, many of whom where thought to be holidaymakers from Karachi.

"It is a very gruesome situation," said police official Aga Mohammed Tahir.

One witness told AFP news agency: "It's a painful scene. There are bodies scattered all over.

"People are crying, fathers are looking for children, husbands for their wives and brothers for their sisters."

The accident happened at about 0400 (2300 GMT Tuesday) on the border between the provinces of Sindh and Punjab.

The express train heading from Lahore to Karachi slammed into the rear of broken down Quetta Express at a station near Ghotki, about 600km (370 miles) north-east of Karachi.

A third train travelling in the opposite direction, heading from Karachi to Rawalpindi - the Tezgam Express - then hit a number of derailed carriages, which were scattered over several tracks.

Abdul Wahab Awan, general manager of Pakistan Railways, said the conductor of the Karachi Express had failed to read a signal correctly.

The Karachi train was travelling at 120km/h (75mph) when it struck the stationary train, a senior railway official told AFP.

The injured have been taken to hospitals in three nearby towns. A doctor at a hospital in Sukkur said about a dozen people there were in a critical condition. Some of the injured had lost limbs or suffered terrible head injuries.

Pakistani President Pervez Musharraf said he was "deeply saddened" at the crash and ordered an urgent investigation.

Pakistan is no stranger to train crashes involving multiple deaths. The BBC's Paul Anderson says casualty figures are often so high because trains are packed with far greater numbers than they were designed for.

More than 200 people were killed in a train crash in Sindh province in 1990. The following year more than 100 people were killed in another accident, also in Ghotki.