India's president and prime minister have appealed for calm after a series of explosions killed at least 38 people in the western city of Ahmedabad.
President Pratibha Patil urged people to "remain steadfast in this testing time and maintain peace and harmony".
Ahmedabad was the scene of religious violence between Hindus and Muslims in early 2002 which left hundreds dead.
On Saturday, 17 blasts within an hour struck residential areas, market places, public transport and hospitals.
It is thought the explosions were caused by crudely-made devices containing ball-bearings and other shrapnel, hidden in boxes and on bicycles. Local media reports say a little-known Islamist group called the Indian Mujahideen has claimed responsibility.
The attack in Gujarat state's commercial capital came a day after several devices went off in the southern city of Bangalore.
The government has put the security agencies on alert in other major cities, including Mumbai, Delhi and Jaipur, some of which have been targets of serial bomb explosions in the past.
The bombs in Ahmedabad were detonated with timers and all went off in the space of 36 minutes, the first at about 1830 (1300 GMT), officials said.
The BBC's Damian Grammaticas in Delhi says the attacks appear to have been a planned and highly co-ordinated, with some targeting the hospitals where many of the injured were being treated. More than 100 people were wounded.
A BBC correspondent in Ahmedabad says many people are not leaving their houses because of fear of further attacks.
In a statement on her website, President Patil "expressed her heart-felt condolences for the loss of life and urged the people of Ahmedabad to remain steadfast in this testing time and maintain peace and harmony".
Prime Minister Manmohan Singh also condemned the attacks, and urged people to remain calm and maintain communal harmony.
Narendra Modi, the controversial chief minister of Gujarat, said the " land of Mahatma Gandhi has been bloodied by terrorists whom we shall not spare".
"Terrorists are waging a war against India. We should be prepared for a long battle against terrorism," he warned.
Mr Modi has been accused of failing to protect Muslims in the riots in Gujarat during 2002 in which 1,000 people died, including many in Ahmedabad. The violence was sparked by an attack on a train carrying Hindu pilgrims. (BBC)