Three bombs have rocked crowded areas of Mumbai during the evening rush hour, killing at least 21 people in the biggest such attack on India's financial capital since the 2008 assaults blamed on a Pakistan-based group.
At least 141 people were wounded in Wednesday’s blast, officials said.
"The blast occurred at about 6.45 pm (13:15 GMT) within minutes of each other. Therefore, we infer that this was a coordinated attack by terrorists," Palaniappan Chidambaram, the federal home minister said.
There was no immediate claim of responsibility for the blasts and Indian officials refused to speculate on who might be behind them.
At least one car and a motorbike were used in the attacks in which improvised explosive devices were believed to have been used, officials said.
The biggest blast was at the Opera House, an area in south Mumbai which is a hub for the diamond trade. The bomb was most likely kept in an umbrella, said Arup Patnaik, the Mumbai police chief.
Another blast, also in south Mumbai, was at the Zaveri Bazaar, India's largest bullion market, which was hit twice in the past.
The third blast was at Dadar, in the centre of the city and a busy intersection on Mumbai's sprawling suburban train network.
The blasts marked the first major attack on Mumbai since 10 gunmen laid siege to the city for 60 hours in November 2008. That attack killed at least 166 people and targeted a busy train station, two luxury hotels and a Jewish centre.
C. Uday Bhaskar, a defence analyst, said the bombings showed that Mumbai remained vulnerable despite precautions taken after the 2008 attack.
"The local police still does not have either the capability or the capacity to pre-empt such attacks, and this is going to be a constant challenge," he said.
In December, authorities deployed extra police on city streets after receiving intelligence that a Pakistan-based group was planning an attack over New Year's weekend. In September, police issued a terror alert for the city during a popular Hindu festival.
In March 2010, Mumbai police said they prevented a major strike after they arrested two Indian men, who, police said, were preparing to hit several targets in the city.
Mumbai has been the target of multiple blasts before.
On March 12, 1993, the 13 serial bombs that shattered Mumbai killed 200 people and wounded more than 1,000.
In July 2006, bombs exploded in the city's rush-hour trains and stations, killing 187 people and wounding more than 700.
Source: Al Jazeera and agencies