Indian police are searching for clues into who was behind three co-ordinated bomb blasts that killed at least 17 people in Mumbai in an evening rush hour attack.

Palaniappan Chidambaram, India's home minister, said on Thursday that all groups "hostile to India" were being treated as possible suspects behind the attacks.

The government's official press office lowered the death toll from Wednesday's attacks to 18 from an earlier figure of 21 killed, although the number may change again.