Video | India on high alert after Mumbai blasts kill 17

As Indian Prime Minister Manmohan Singh appealed to the people of Mumbai "to remain calm and show a united face", India's Home Minister P Chidambaram expressed suspicions that the blasts were "a co-ordinated attack by terrorists", as the explosions occurred within minutes of one another.

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. All three bombs were reported within a roughly 15-minute period, starting at around 1850 local time (1320 GMT).

They were placed in a bus stand, on the road, and on a motorcycle respectively. Despite how the explosion in the Zaveri Bazaar area was of low intensity, the blasts at Opera House and Dadar were "of medium to high density", Chidambaram said.

The biggest explosion occurred at the Opera House in an area known as a hub for diamond traders.

On Thursday, the government revised the death toll from the blasts down from 21 to 17, and said 131 people had been injured.

But rescue workers also found a severed head at one of the blast sites which had not yet been identified or included in casualty figures, Chidambaram said.

No group has said they carried out the attack, which took place in three districts during evening rush hour.

"All groups that have capacity to carry out such terror attacks are suspect. All angles will be investigated, all leads will be followed. I appeal to people not to speculate," he said.

Federal commando, forensic and investigation teams have arrived in Mumbai to help the local police.

The capital, Delhi, Calcutta and several other cities have also been put on alert, with a police presence being stepped up at public places like malls, cinemas, parks and transport terminals.

Peter Wittig, the current president of the UN Security Council, said members of the UN body condemned the attack "in the strongest terms".

"They expressed their deep sympathy and sincere condolences to the victims of these heinous crimes and acts and to their families and to the people and government of India," he said.

"The Security Council is determined to combat all forms of terrorism," he added.

One witness said he had tried to help by getting the wounded onto motorbikes to take them to hospital.

Most of Mumbai, however, began to return to normal life as dawn broke on Thursday, with vendors making their usual rounds and schools kept open despite the attack.

Mumbai has been targeted many times in recent years.

The 2008 attacks, which targeted two high-end hotels, a busy train station, a Jewish centre and other sites frequented by foreigners, were blamed on the Pakistan-based Lashkar-e-Taiba militant group.

Pakistan was quick to condemn the latest explosions, in a statement issued by the foreign ministry.

Peace talks between Pakistan and India have only recently resumed since they were broken off after the 2008 attacks.

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.