Ahmedabad, home to 3.5 million people and Gujarat’s commercial center, is no stranger to violence. In 2002, a train fire that killed several dozen Hindus led to the killing of 1,000 Muslims over several days, one of the worst outbreaks of religious violence in India’s history.

An obscure group calling itself the Indian Mujahedeen warned Saturday that an attack was about to take place “in revenge of Gujarat,” plainly referring to the 2002 killings. The statement was sent in an e-mail message, written in English, to television stations just before the first blasts.

H. P. Singh, the city’s joint police commissioner, said Sunday that some of the explosives had been strapped to bicycles in crowded streets and markets. Later in the evening, a pair of car bombs went off in front of two city hospitals. At one of them, Civil Hospital, the dead included husband-and-wife doctors and two sanitation workers.

The police said two additional bombs had been found and defused, in Ahmedabad and nearby Gandhinagar, Gujarat’s capital. On Sunday afternoon, the police found two abandoned cars in Surat, an industrial city in Gujarat, one stuffed with bomb-making chemicals and detonators, the other with live bombs. The police said they were still tracing the cars’ ownership.

On Friday, there was a series of similar low-intensity blasts in southern Bangalore, one of which killed a woman standing at a bus stop. Two months ago in Jaipur, synchronized blasts on bicycles killed 56 people; the Indian Mujahedeen sent an e-mail message claiming credit for those attacks.

On Sunday, a police official, P. P. Pandey, said “a single mind” was suspected to be behind the three latest attacks. The police said they had detained people for questioning; The Associated Press reported 30 were in custody. Officials offered no further details about who was involved in the group or a possible motivation behind the bombings.