Two passenger trains were derailed in separate incidents in India on Sunday, leaving at least 35 people dead and about 200 injured in a day of carnage on the country's railways.

In the first accident in the northern state of Uttar Pradesh, a packed train travelling at full speed was thrown off the tracks, killing 35 passengers as twisted carriages were forced on top of each other.

Later, an explosion hit another train in the northeastern state of Assam, but police were unable to say whether the blast was caused by separatist militants active in the area.

About 100 people were injured in each of the derailments, officials said, with rescue teams and local residents trying to free people trapped inside the carriages.

"We were sitting in our seats when suddenly everything turned upside down," a male passenger interviewed by the CNN-IBN news channel said at the scene of the earlier derailment in Uttar Pradesh, 150 kilometres (95 miles) south of Lucknow city.

"When the train stopped we broke the glass windows to jump out on the track."

K.N. Joshi, the district chief medical officer for nearby Fatehpur, told AFP that some victims remained in the carriages as night fell.

The Press Trust of India news agency reported the driver was among the injured and that the train, carrying about 1,000 people, was moving at near its top speed of 108 kilometres an hour when it derailed.

The Kalka Mail was heading from Howrah, the main station for the eastern city of Kolkata, across India to the capital New Delhi when it left the tracks.

Emergency teams were still working in the dark to cut their way into two carriages to search for survivors. Officials said the death toll could rise and that an investigation had been launched.

Prime Minister Manmohan Singh "expressed deep sorrow and shock at the loss of lives" and promised all available resources would be deployed for rescue and relief operations, his office said in a statement.

Anxious relatives and friends of the passengers gathered at Howrah and other stations along the line seeking information about their loved ones.

At the incident in Assam state, no fatalities were reported but 20 of the 100 injured were described by police as being in a serious condition.

The Guwahati-Puri Express was nearing Ghograpara, about 70 kilometres from Assam's main city of Guwahati, when the blast struck.

"There was a loud explosion and it was total chaos soon after," Jiten Das told AFP by telephone.

"The coach in which I was travelling skidded off the track and fell in marshy land with waist-deep water. Somehow we managed to get out. I cut my head and arms and have a wound in my chest."

Both the outlawed United Liberation Front of Asom (ULFA) and the National Democratic Front of Bodoland (NDFB) militants are active in the region, but the cause of the explosion was not known.

Only three days ago, 38 people were killed in a rail crash in Uttar Pradesh when a train slammed into a bus carrying a wedding party.

India's state-run railway system -- still the main form of long-distance travel despite fierce competition from new private airlines -- carries 18.5 million people daily.

The worst accident in India was in 1981 when a train plunged into a river in the eastern state of Bihar, killing an estimated 800 people.

The railway is the country's largest employer with 1.4 million people on its payroll and it runs 11,000 trains a day.

Experts say the creaking system, the world's second largest under a single management, is desperately in need of new investment to improve safety and help end transportation bottlenecks that threaten the country's economic growth.