Indian Prime Minister Manmohan Singh on Thursday vowed that those responsible for a series of bomb blasts that killed 17 inMumbai would be tracked down and prosecuted, as he visited the injured.

"Perpetrators of (the) Mumbai blasts shall be pursued relentlessly and brought to justice quickly," the premier said on a tour of city hospitals treating some of the more than 130 wounded in Wednesday's blasts.

Investigators are banking on security camera footage to pinpoint who was behind the wave of explosions, which happened within 15 minutes in two crowded commercial areas of south Mumbai and a central residential district.

Police said that the bombs were made of ammonium nitrate -- an ingredient for fertiliser commonly used in improvised devices.

Home Minister P. Chidambaram earlier said there had been no intelligence of an impending attack and in the absence of any group claiming responsibility, the net of suspicion was flung far and wide.

"All groups hostile to India are on the radar. We are not ruling out anything, we are not ruling in anything. We are looking at everyone," he said after visiting the scene of the three blasts.

"I think they chose the places because of the density of the population and the very congested nature of these areas," he added. "They chose places where even a low-intensity blast could have a great impact."

Chidambaram's department said early on Thursday evening that 13 people were in a critical condition.

Specialist forensic teams, flown in from other cities, combed the blast sites for evidence, but Rakesh Maria, head of the Maharashtra state anti-terrorism squad, said monsoon rains were hampering their work.

Maria declined to speculate on who might be responsible for the blasts and said investigators hoped security camera footage images obtained from all three locations would provide them with a lead.

Crime branch officers have been looking at the footage since last night.

"It's quite a long-drawn process," Maria said, while appealing for public "faith and trust" in the police.

"No matter where the accused are, we will identify and bring them to book," he said.

The Home Ministry said police were interrogating suspected members of the homegrown militant group Indian Mujahideen who were arrested in Mumbai several days ago in connection with bomb blasts in the western state of Gujarat in 2008.

The strongest of Wednesday's coordinated explosions hit busy jewellery trading districts in south Mumbai, close to the same area targeted in the traumatic 2008 assault blamed on the Pakistan-based Lashkar-e-Taiba militant group.

The memory of 60 hours of mayhem when 10 gunmen rampaged through the main railway station and luxury hotels, killing 166, is still fresh in the minds of Mumbaikars.

Salim Dharolia, a diamond trader who was waiting to collect the body of his son at the Saifee hospital, asked why more blood had been shed in his city.

"I have lost my only son. He got married two months ago," he told AFP. "Why are people of Mumbai being targeted all the time? What is our crime?"

Relatives gathered at the 13 city hospitals where victims were transferred in ambulances, cars and trucks driven by locals who rushed to help.

Among the dead was real estate agent R.K. Shah, 47, whose distraught wife had identified his body at Saifee hospital.

"He was scheduled to show two shops for rent to his clients," Pratika Shah told AFP. "Before leaving home, he had told me that he would host a big party for his friends if the deals materialised.

"I still can't believe that he is dead," she said.

The blasts came before a visit to India by US Secretary of State Hillary Clinton next Tuesday and peace talks between the Indian and Pakistani foreign ministers scheduled for New Delhi in the last week of July.

India's foreign ministry spokesman said the talks would go ahead.

The United States and the United Nations led international condemnation of the attacks, with Clinton calling them "despicable" and vowing to go ahead with her trip.

The last major bombing in India was in February last year in Pune, when a blast at a packed restaurant killed 16 people including several foreigners.

In 2006, a series of high-powered blasts on suburban trains in Mumbai killed 187 commuters and left 800 injured -- an attack that India also blamed on Pakistan-based militants.

India broke off a peace dialogue with Pakistan after the 2008 assaults and talks between the two nuclear-armed rivals only resumed earlier this year.