"It is another attack on the heart of India, another attack on Mumbai," said the chief minister of the state, Prithviraj Chavan, who hinted at possible foreign involvement by saying it was a "challenge to Indian sovereignty."


The Home Ministry said at least 20 people had been killed and 113 injured, many of them seriously.


Home Minister P. Chidambaram told reporters in New Delhi that it was "a coordinated attack by terrorists," adding that "the entire city of Mumbai has been put on high alert."


The targets included a predominantly middle class residential area, a wholesale gold market, and a building housing diamond traders and jewellery shops.


All three explosions went off within a 15-minute period, starting at around 6:50pm (1320 GMT).


"It is clear that the attackers wanted to hurt as many as people as possible. A lot of people are injured," a minister in the Maharashtra state government, Chhagan Bhujbal, told reporters.


No group claimed responsibility for the attack, though suspicions initially fell on two Islamist groups that have targeted India in the past: the home-grown Indian Mujahideen and the Pakistan-based Lashkar-e-Taiba (LeT).


US President Barack Obama was among the first to condemn the bombings.


In November 2008, 10 militants from the LeT attacked multiple targets in Mumbai, including five-star hotels, in an assault known as "26/11" in India and often compared to the September 11 attacks on the United States.


India broke off a peace dialogue with Islamabad afterwards, blaming it for failing to crack down on the militant group. Talks between the two nuclear-armed rivals only resumed earlier this year.


Eyewitnesses outside the diamond trader building in south Mumbai said a car bomb had exploded when the area was packed with office workers heading home.


"There were lots of people badly injured. We don't know how many are dead but it was a very big blast," Nimesh Mehta, 38, who runs a local food stall, told AFP.


Jayesh Labdhi, committee member of Mumbai Diamond Merchants Association, said he heard a massive explosion, after which the site was covered in smoke.


"There were so many people lying on the ground just crying out and wanting help," he said.


The injured, many of whom had serious injuries, were bundled into taxis or hauled onto tarpaulin sheets and taken to hospital, he added.


The last major bombing in India was in February last year in the western city of Pune, when a blast at a packed restaurant killed 16 people including several foreigners.


In 2006, a series of seven high-powered blasts on suburban trains in Mumbai killed 187 commuters and left 800 injured — an attack that India also blamed on Pakistan-based militants.

Mumbai rocked by three explosions at least 20 feared dead