Indian cities are on a state of high alert after three blasts shook the commercial hub Mumbai, killing 18 people and hurting dozens.
Indian PM Manmohan Singh has appealed to the people of Mumbai (Bombay) "to remain calm and show a united face".
No group has said they carried out the attack, which took place in three districts during the evening rush hour.
The attacks are the deadliest in Mumbai since November 2008, when gunmen killed 165 people in a three-day raid.
Pakistani-based militants were blamed for the 2008 attacks - and the government in Islamabad was quick to condemn the latest attacks.
The United Nations also condemned the attack, describing it as "heinous" and pledging to "combat all forms of terrorism".
'All leads followed'
India's Home Minister P Chidambaram has said he believed the blasts were "a co-ordinated attack by terrorists", as the explosions occurred within minutes of one another.
But he said it was too early to speculate on who might have been behind the attack.
"All groups that have capacity to carry out such terror attacks are suspect. All angles will be investigated, all leads will be followed," he told reporters.
Mr Chidambaram said no intelligence had been received about a coming attack, and suggested that those responsible had "worked in a very clandestine manner - maybe a very small group that has not communicated with each other".
All three bombs were reported within a 15-minute period, starting at around 1850 local time (1320 GMT).
The biggest explosion occurred at the Opera House business district, in an area known as a hub for diamond traders.
One witness said he had tried to help by getting the wounded onto motorbikes to take them to hospital.
"We came outside, and the area was filled with black smoke. There were bodies lying all over the street, there was lots of blood... We saw many bodies missing arms and missing legs," Aagam Doshi told Reuters news agency.
Another blast hit the Dadar district in the city centre, known for its gold market, while a third, described by the authorities as low intensity, hit the Zaveri Bazaar.
Mr Chidambaram said none of the bombs had been triggered remotely.
Federal commando, forensic and investigation teams have arrived in Mumbai to help the local police.
It is hoped security cameras at the gold market and jewellery shops where the two biggest blasts occurred will aid the investigation
The 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.
Schools open
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, Mr Chidambaram said.
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.
The gunmen, nine of whom died in the raid, killed 165 people.
Peace talks between Pakistan and India were broken off after the attacks and have only recently resumed.
In a statement on Wednesday, Pakistan's President Asif Ali Zardari and Prime Minister Yousuf Raza Gilani expressed their "deepest sympathies to the Indian leadership".
