Bombs exploded on packed commuter trains and stations in Mumbai on Tuesday, killing over 160 people and wounding hundreds, officials said.
There was no immediate claim of responsibility for the seven bomb explosions that took place within about 10 minutes during evening rush hour.
But suspicion was likely to centre on Muslim militants fighting New Delhi's rule in disputed Kashmir, who have been blamed for several bomb attacks in India in the past.
"The death toll is 163 and around 460 people have been injured," police inspector Ashok Jadhav told Reuters.
"We are not sure if it is RDX or not," city Police Commissioner A.N. Roy said, referring to the possible use of high-powered plastic explosives.
Commuters fled suburban rail stations in panic after the explosions and mobile phone lines were jammed. Hundreds of dazed passengers walked along the railway tracks.
Television showed twisted rail carriages and people in torn, blood-stained clothes carrying the dead and wounded on stretchers as steady monsoon rain fell. A policeman was shown carrying two white, blood-stained bundles of what appeared to be body parts.
"The blasts happened when the trains were most crowded," D.K Shankaran, chief secretary of Maharashtra, told Reuters.
At peak hours, each nine-car passenger train in Mumbai carries over 4,500 people, about three times the rated capacity.
Prime Minister Manmohan Singh called for calm and Sonia Gandhi, leader of the ruling Congress party, expressed her grief.
"I urge the people to remain calm, not to believe rumours and carry on their activity normally," Singh said in a statement, calling the explosions a "shameful act".
The United States called the bomb attacks "senseless acts of violence". Pakistan, the EU, France and Britain also condemned the explosions.