More than 160 people have been killed and 460 injured by seven bombs on the train network in the Indian financial capital Mumbai (Bombay), police say.
Rescue efforts have been hampered by heavy rain
The first of the near-simultaneous blasts went off at about 1830 local time (1300 GMT), during the rush hour in the suburbs on the Western Railway.
Correspondents spoke of scenes of pandemonium, with people jumping from trains and bodies flung onto tracks.
There have been a number of bomb attacks in Mumbai in recent years.
The city and the capital Delhi have been put on high alert, and Mumbai's entire rail network has been shut down, stranding hundreds of thousands of commuters.
Indian Prime Minister Manmohan Singh appealed for calm and described the incidents as a "shocking and cowardly attempt to spread a feeling of hatred".
Pakistani leaders also condemned the blasts as a "despicable act of terrorism".
Police said the co-ordinated blasts took place at Matunga, Khar, Mahim, Jogeshwari, Borivali and Bhayandar, with most on moving trains and two at stations.
Local reports said the bombs appeared to have targeted first-class compartments, as commuters were returning home from the city's financial district.
A shopkeeper near one explosion said it was so powerful they thought they had been hit by lightning.
Television images show dazed and bloodied commuters being carried by fellow passengers to waiting ambulances, as rescue workers clambered through wreckage to reach victims.
The force of the blasts ripped doors and windows off carriages, and scattered luggage.
Clothes and shoes were strewn along the tracks.
An eyewitness at Mahim told the BBC some of those who had jumped from the train were run over by another train coming in the opposite direction.
Hospitals across Mumbai have been swamped with casualties.
A medical student at a hospital in Parel, which has received many of the wounded, told the BBC News website the "floors are filled with bloodstains".
"There were so many [injured people], I couldn't really count," Sunny Jain said.
Correspondents say the confusion in the city has been made worse by the collapse of mobile phone networks, and long queues have formed for the payphones still working.
There was no immediate claim of responsibility for the attacks, which are the worst in the city for more than a decade.
Changed named to Mumbai in late 1990s
Population: 17 million
HQ to many big Indian companies and foreign multinationals
Home of the Indian film industry, Bollywood
Thousands arrive monthly from rural areas, with many living in slums
Property prices are among the most expensive in the world
At least 250 people died in serial bomb blasts in the city in 1993
Home Minister Shivraj Patil told reporters authorities had "some" information an attack was coming, "but place and time was not known".
Analysts say Mumbai has been a repeated target because it is a financial hub and a centre for the underworld.
More than 250 people died in a string of blasts in the city in 1993.
Correspondents say Tuesday's bombers could hardly have struck a target with greater impact - both practical and psychological.
The city's suburban train system is one of the busiest in the world, carrying more than six million commuters a day.
The blasts came hours after suspected Islamic extremists killed seven people in grenade attacks in the summer capital of Kashmir, Srinagar.