Home Minister P. Chidambaram said he believed Wednesday’s rush-hour attacks in India's commercial hub were "a co-ordinated attack by terrorists," as the explosions occurred within minutes of one another.
But he refused to speculate about who was behind the assaults saying authorities had received no prior warning or claim of responsibility.
"All groups hostile to India are on the radar. We are looking at everyone," he told reporters after visiting the scene of the blasts.
The bombs went off within 15 minutes of each other in two crowded commercial areas of south Mumbai and a central residential district.
The official death toll stood at 17, he said, though it is expected that that figure will increase. A total of 131 injured were admitted to hospital, of which 23 were in a serious condition.
Wednesday evening’s attacks were the deadliest on Indian soil since the 2008 assault on Mumbai, blamed on the Pakistan-based Lashkar-e-Taiba militant group, which left 166 dead.
Chidambaram said that all cities in the emerging Asian nation were vulnerable to attacks.
"We live in the most troubled neighbourhood in the world. Pakistan-Afghanistan is the epicentre of terror," he said.
India's capital, Delhi, Calcutta and several other cities have been put on alert, with a police presence being stepped up at public places like malls, cinemas, parks and transport terminals.
EU foreign policy Chief Catherine Ashton joined in international condemnation of the assaults on Thursday, denouncing the "heinous" attacks and calling for the perpetrators to be brought to justice.