Three near-simultaneous explosions have shaken India's commercial capital Mumbai (Bombay), police say.
Twenty-one people were killed and 113 injured, said Maharashtra state's Chief Minister, Prithviraj Chavan.
He called the explosions, during Mumbai's busy evening rush-hour, "a co-ordinated attack by terrorists".
One explosion was reported in the Zaveri Bazaar, another in the Opera House business district and a third in Dadar district in the city centre.
Police sources were reported as saying the explosions were caused by home-made bombs.
The attacks are the deadliest in Mumbai since November 2008 when 10 gunmen launched a three-day co-ordinated raid in which 166 people were killed.
Prime Minister Manmohan Singh denounced the bombings and appealed to Mumbai residents "to remain calm and show a united face".
US President Barack Obama strongly condemned the "outrageous" attacks, and offered "support to India's efforts to bring the perpetrators of these terrible crimes to justice".
The latest explosions hit the city as workers were making their way home.
The first struck the Zaveri Bazaar at 1854 (1324 GMT), tearing through the famed jewellery market, according to police. A minute later, a second blast hit the busy business district of Opera House, in the south of the city. At 1905, the third bomb exploded in the Dadar area of central Mumbai.
Because the explosions occurred within minutes of each other, "we infer that this was a co-ordinated attack by terrorists", Home Minister Palaniappan Chidambaram told reporters.
Mumbai had been put on a state of high alert and a commando team was standing by, he said. Delhi, the capital, Calcutta and several other cities have also been put on alert.
Forensics teams have been sent from Delhi and Hyderabad to examine the explosion sites.
The authorities have not yet said who they believe might be behind the explosions and no group has said it carried them out.
In Zaveri Bazaar, witnesses described a motorcycle exploding next to a jewellery shop. Mumbai Police Commissioner Arup Patnaik said a bomb had been left in an abandoned umbrella.
Photographer Rutavi Mehta told the BBC he was shopping nearby and heard the explosion. He grabbed his camera and ran to the scene.
"I took a couple of photographs. I think they might be too graphic for broadcast," he said.
"Bodies and limbs were strewn everywhere. People were crying and screaming. The area was packed with shoppers at the time of the blast. A few offered assistance to the blood-soaked victims, while others looked on in a state of shock," he said.
"It was totally chaos. There were pools of blood everywhere."
The second and most powerful blast was in the nearby Opera House district. Local media said it was planted inside the two-storey Prasad Chamber building.
In the central Dadar district, the bomb tore apart a taxi that was parked next to a bus stop, witnesses told the BBC. It was unclear whether the explosives were planted inside the vehicle or in a nearby electricity meter box.
"I heard a loud explosion. And then I saw people with serious injuries lying in pools of blood," another person told the Times of India.
An unexploded bomb was also reportedly been found in Dadar.
The choice of locations makes it clear that the blasts were intended to cause maximum casualties, says the BBC's Soutik Biswas in Delhi.
But footage of one of the blast sites - a ripped-off cover of a bus shelter and a car with its glass shattered - points to a medium-level and possibly crude explosion, adds our correspondent.
According to some reports, the blasts came on the birthday of Mohammad Ajmal Amir Qasab, the sole surviving gunman from the 2008 attacks. But court records show his birthday to be in September.
Those attacks, which targeted two high-end hotels, a busy train station, a Jewish centre and other sites frequented by foreigners, were blamed on the Pakistan-based Lashkar-e-Taiba militant group.
Pakistan was quick to condemn the latest explosions, in a statement issued by the foreign ministry.
Peace talks between Pakistan and India have only recently resumed since they were broken off after the 2008 attacks.
Mumbai has been targeted many times in recent years.
As well as the 2008 attacks, co-ordinated blasts on seven of the city's trains on 11 July 2006 caused massive loss of life. More than 180 people were killed and hundreds wounded in those bombings, which were blamed on Islamist militants.
The city suffered four bomb attacks during 2003, including twin blasts on 25 August 2003 which killed 52 people.
In 1993, 257 people were killed and 700 injured in a series of 12 bomb blasts across the city. The attacks were allegedly ordered by the Muslim-dominated underworld in retaliation for Hindu-Muslim riots.