No group has claimed responsibility for the attacks, the most deadly since Pakistan-based militants struck India's financial hub in 2008, killing 166 people and raising tensions with nuclear rival Pakistan.

Indian authorities have yet to say publicly who they believe was responsible for the three near-simultaneous blasts during the evening rush hour, which killed 18 people and injured 133 others.

The blasts have heaped pressure on Singh as he struggles to overcome a series of graft scandals that have boosted a resurgent opposition and led to policy paralysis in Asia's third largest economy.

"The terrorists had the advantage of surprise," Singh said in rare public comments outside a hospital after meeting some of the injured. "This time there was no advance indication.

"Now our task is to find out who the culprits are and how we can work together to bring them to justice," he said.

India's main opposition, the Hindu nationalist Bharatiya Janata Party (BJP), accused the government of being lax.

"These repeated attacks on Bombay (Mumbai) should be viewed as a policy failure. It is not an intelligence failure," said top BJP leader L.K. Advani, a former deputy prime minister.