Seven explosions rocked Mumbai's commuter rail network during today's evening rush hour, killing at least 105 people and injuring 300, officials said.
India's major cities were put on high alert after the blasts.
Chaos erupted throughout Mumbai's crowded rail network following the explosions, and authorities struggled to determine how many people had been killed and injured.
Mumbai Police Chief AN Roy said on Indian television 100 people were feared killed and more than 300 injured.
"We are busy in the rescue operation. Our first priority is to rescue the injured people," he said.
However, heavy monsoon downpours were hampering the effort.
Indian Prime Minister Manmohan Singh called an emergency Cabinet meeting, and said that "terrorists" were behind the attacks.
Home Minister Shivraj Patil told reporters that authorities had "some" information an attack was coming, "but place and time was not known."
Vilasrao Deshmukh, the chief minister of Maharashtra state, where Mumbai is located, said bombs had caused the blasts.
"The official information is that these are bomb explosions," the minister said.
Police were also reportedly carrying out raids across the country following the explosions, presumably in search of suspects.
One television report said a suspect was in custody.
A senior Mumbai police official, PS Pasricha, said the explosions were part of a well-co-ordinated attack.
Witnesses reported seeing body parts strewn about stations, and Indian television news channels broadcast footage of bystanders carrying victims to ambulances and searching through the wreckage for survivors and bodies.
Some of the injured were seen frantically dialling their mobile phones.
The force of the blasts ripped doors and windows off carriages, and luggage and debris were strewn about, splattered with blood.
Survivors were seen clutching bloody bandages to their heads and faces. Some were able to get up and walk from the stations.
Pranay Prabhakar, the spokesman for the Western Railway, confirmed that seven blasts had taken place.
He said all trains had been suspended in Mumbai, and he appealed to the public to stay away from the train stations in the city of 16 million, India's financial and commercial centre and principal port on the Arabian sea.
There was no immediate claim of responsibility for the attacks, but the blasts came in quick succession - a common tactic employed by Kashmiri militants that have repeatedly targeted India's cities.
The first explosion hit the train at a railway station in the north-western suburb of Khar, said a police officer.
India's CNN-IBN television news, which had a reporter travelling on the train, said the blast took place in a first-class carriage as the train was moving, ripping through the compartment and killing more than a dozen people.
Roy, the Mumbai police chief, said the other blasts targeted trains, railway tracks and platforms.
One explosion occurred near an underground station.
The purported attacks came hours after a series of grenade attacks by Islamic extremists that killed eight people in the main city of India's part of Kashmir.
Dozens of militant groups have been fighting Indian rule in Kashmir, demanding the mostly Muslim region's independence, or its merger with Pakistan.
India accuses Pakistan of aiding the militants, who run training camps on Pakistan's of Kashmir. Pakistan denies the charge.
India and Pakistan have fought three wars since the subcontinent was partitioned upon independence from Britain in 1947, two over Kashmir.
Accusations of Pakistani involvement in a 2001 attack on India's parliament put the nuclear-armed rivals on the brink of a fourth war.
But since then, Pakistan and India embarked on a peace process aimed at resolving their differences, including their conflicting claims to all of Jammu-Kashmir.
The Pakistani Foreign Ministry issued a statement today condemning the attacks.
"Pakistan strongly condemns the series of bomb blasts on commuter trains," the ministry said in a statement.
President Gen Pervez Musharraf and Prime Minister Shaukat offered condolences over the loss of life, the statement said, adding "terrorism is a bane of our times and it must be condemned, rejected and countered effectively and comprehensively."Reuse content