Officials said at least 464 people were injured in the blasts in the city's western suburbs as commuters made their way home. All seven blasts came within an 11-minute span, between 6:24 and 6:35 p.m. (12:54 and 1:05 p.m. GMT).
Analysts are comparing the attack with the mass transit bombings in Madrid in 2004 and London last year, saying they all involved a series of mutiple blasts and were well-coordinated.
There was some confusion about the number of dead and injured as information was compiled from hospitals and explosion sites in Mumbai, the west Indian seaport previously called Bombay.
"There still are bodies being recovered," said Pooja Saxena, with the International Federation of the Red Cross, speaking early Wednesday.
CNN-IBN correspondent Jency Jacob was aboard one of the trains during the attacks.
"People started running helter-skelter and started jumping from the train," Jacob said.
"When I jumped from the train, I saw that the first-class compartment was totally ripped apart and people were hanging from the train. There are some people who were thrown out from the train and they were lying on the track, bleeding completely."
One person was arrested in New Delhi in police raids after the explosions, reported CNN-IBN, CNN's sister network, but there's been no claim of responsibility for the attacks.
Indian Prime Minister Manmohan Singh urged calm and said the attacks were "shocking and cowardly attempts to spread a feeling of fear and terror."
"I reiterate our commitment to fighting terror in all its forms," he said in a written statement.
U.S. officials said suspicion fell on two Islamic terrorist groups whose focus has been on the disputed territory of Kashmir -- Lashkar-e-Tayyiba and Jaish-e-Mohammed.
Both groups have been implicated in attacks that involved coordinated bombings during peak times in India, the officials said.
It may be no coincidence that the attacks occurred just ahead of the Group of Eight summit of world leaders that begins Saturday in St. Petersburg, Russia, said Sajjan Gohel, director of international security for the Asia-Pacific Foundation.
Seven explosions took place on rush-hour trains and stations in Mumbai, formerly called Bombay
Last year's July 7 terror bombings in London that killed 52 people came as UK Prime Minister Tony Blair was hosting the G8 summit in Scotland and one day after London was awarded the 2012 Olympics, Gohel told CNN International.
Both the 2005 London bombings and the 2004 Madrid bombings, that killed 191 people, were directed against rush hour commuters on mass transit systems.
"This time again, they're (terrorists) trying to show that they are live, active. They want attention, they want the focus," Gohel said. "It was a coordinated, multiple, simultaneous mass casualty atrocity. This is the hallmark of a powerful transnational group."
Gohel noted that at least one of Tuesday's attacks targeted a first-class commuter car, and police were looking at that carriage to see if it might yield clues. The names of those aboard would have been known beforehand -- as opposed to regular computers.
Dana Dillon, a senior policy analyst in the Heritage Foundation's Asian Studies Center, said if Indians believed a Pakistani militant group was behind the bombings, it could disrupt two and a half years of dialogue between the countries that has led to a de-escalation of troops and other positive moves.
"If this terrorist attack messes that up, it could be catastrophic to the region," Dillon said.
The blasts hit trains or platforms at the Khar, Mahim, Matunga, Jogeshwari, Borivili and Bhayander stations. The seventh explosion struck a train between the Khar and Santacruz stations, a police official told CNN-IBN.
Police also found and defused another bomb at the Borivili station, according to CNN-IBN.
Video footage from a train station showed people in bloodstained clothes receiving medical treatment, while others were carrying victims and some lying motionless near railroad tracks. Windows of a train appeared to be spattered with blood.
At least one train was split in half.
Jacob said after his train was attacked he moved toward the back of the train where he "could see some explosives, some pipes that were falling down. The police were investigating that. It seems to be that the explosive was packed off in pipes and kept in the first class men's compartment."
A CNN-IBN correspondent who was on one of the trains said it was leaving a station when the blast occurred. People jumped and were killed as the train hit them.
"Limbs [are] lying everywhere, bodies [were] cleared from the tracks by local business owners who rushed from their shops," the correspondent said.
Another CNN-IBN correspondent reported seeing 15 bodies at the Matunga station.
People living almost two miles (three kilometers) away from the Borivili station said they heard the blast.
The Western Railway system -- which 4.5 million people use daily -- was shut down and Mumbai's subway system put on high alert after the blasts. Police in the capital of New Delhi also heightened security.
Airports across India were put on high alert, too.
U.S. officials said the blasts followed a pattern of initiated by the two main Islamic Kashmiri separatist terrorist groups.
Kashmiri separatists were blamed for twin car-bombings that killed 53 people in Mumbai in August 2003 as well as an attack on the Indian parliament in Delhi in 2001.
In March 1993, more than 250 people were killed when at least 13 bombs were detonated around Mumbai. That attack followed a wave of fighting between India's Hindu and Muslim communities.
Pakistani President Pervez Musharraf strongly condemned the attacks, and a statement released by his country's Foreign Ministry called them a "despicable act of terrorism."
"Terrorism is the bane of our times and it must be condemned, rejected and countered effectively and comprehensively," the statement said.
Earlier Tuesday, a grenade attack in Indian-controlled Kashmir killed at least four people. Authorities suspect militants are responsible for that attack on a minibus in Srinagar. There was no immediate indication of a connection to the Mumbai blasts.
Indian Home Minister Shivraj Patil said the government had some advance knowledge that such an attack might take place. "What we didn't have was the place and the time," Patil said.