Modi called the blasts "a crime against humanity," and said the state government would cover the medical costs of all those wounded in the attacks.
There was no immediate claim of responsibility for either set of blasts, and it was not clear if they were connected but Modi said that the attacks appeared to be masterminded by a group or groups who "are using a similar modus operandi all over the country."
Distraught relatives of the wounded crowded the city's hospitals and television channels showed video footage of police officers and sniffer dogs scouring the areas that were hit.
There also were images of a bus with shattered windows, destroyed roadside stalls, twisted bicycles and charred vehicles. Most of the blasts took place in the narrow lanes of the older part of Ahmadabad, which is crowded with tightly packed homes and small businesses.
Prithviraj Chavan, a junior minister in the prime minister's office, called the explosions "deplorable" and said they were set off by people "bent upon creating a communal divide in the country" -- language officials usually use when blaming Islamic militants believed to be behind bombings that have repeatedly hit India's cities in recent years.
"Anti-national elements have been trying to create panic among the people of our country. Today's blasts in Ahmadabad seem to be part of the same strategy," federal Home Minister Shivraj Patil told reporters in New Delhi.
Patil provided no details about the explosions.
The latest attacks came a day after seven synchronized small bombs shook Bangalore, India's high-tech hub, killing two people and wounding at least five others.
On Saturday, police found and defused an eighth bomb near a popular shopping mall in Bangalore, said Srikumar, the director general of police in Karnataka state, where the city is located. Like many Indians, he uses only a single name.
As in past bombings in India, suspicion for both sets of explosions quickly fell on Muslim militants blamed for attacks such as the July 2006 bombings that ripped through Mumbai's commuter rail network, killing nearly 200 people.
Those fears were amplified by the history of Ahmadabad, a crowded and historic city that in 2002 was the scene of one of worst incidents of rioting between India's Hindu majority and its Muslim minority.
The violence killed about 1,000 people, most of them Muslims. It was triggered by a fire that killed 60 passengers on a train packed with Hindu pilgrims. Hindu extremists blamed the deaths on Muslims and rampaged through Muslim neighborhoods, although the cause of the blaze remains unclear.
RSS feeds | Slideshow archive | ABC13 wireless | Help solve crimes