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".