India's major cities were put on high alert after the blasts, and Indian Prime Minister Manmohan Singh called an emergency Cabinet meeting, saying "terrorists" were behind the attacks, reported the Associated Press.
Commuters fled suburban rail stations in a panic, and television pictures showed twisted rail carriages and people in bloodstained clothing carrying the dead and wounded. Luggage and debris lay scattered.
On Tuesday evening, Maharashtra state Chief Minister Vilasrao Deshmukh increased earlier death toll estimates from 105 to 147 and said 439 were wounded.
"We are busy in the rescue operation," said Mumbai (formerly Bombay) Police Chief A.N. Roy to the AP. "Our first priority is to rescue the injured people."
There was no immediate claim of responsibility, but suspicion turned to Muslim militants fighting New Delhi's rule in disputed Kashmir. The bombings came in quick succession, a common tactic used by Kashmiri militants.
Officials are also unsure if suicide bombers were responsible for the attacks which occurred after stock markets closed. The first exploded at a railway station in Bandra at around 6:20 p.m. The bombings continued down the line of the Western Railway with the last two both occurring at a station in Borivili at 6:35 p.m.
The Press Trust of India, citing railway officials, said all the blasts had hit first-class cars.
Pakistan's President Pervez Musharraf and Prime Minister Shaukat Aziz issued statements saying they "strongly condemn" the attacks.
India and Pakistan have fought three wars -- two over Kashmir -- since the subcontinent was partitioned after gaining independence from Britain in 1947.
Dozens of militant groups have been fighting Indian rule in Kashmir, demanding independence for the mostly Muslim region or a merger with Pakistan.