Mumbai (Bombay) and other Indian cities have woken up to a state of high alert after three blasts shook the commercial capital, killing 21 and hurting dozens.
Indian PM Manmohan Singh appealed to the people of Mumbai "to remain calm and show a united face".
Later the United Nations strongly condemned Wednesday's attack, describing it as "heinous".
No group has said they carried out the atttack, which took place in three districts during evening rush hour.
Earlier, Maharashtra state's Chief Minister, Prithviraj Chavan said he believed the blasts were "a co-ordinated attack by terrorists", as the explosions had occurred within minutes of one another.
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.
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 15-minute period, starting at around 1850 local time (1320 GMT).
Police sources were reported as saying the explosions were caused by home-made bombs.
Mumbai was put on a state of high alert and a commando team is standing by.
The capital, Delhi, Calcutta and several other cities have also been put on alert, with police 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.
Forensics teams have arrived from Delhi and Hyderabad to examine the explosion sites.
The biggest explosion occurred at the Opera House 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.
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.