A little known group calling itself the Indian Mujaheddin asserted responsibility for the attacks, saying in e-mails sent to several news stations: "Stop us if you can. We strike with message of death. We are terrorizing for your sins."

"This is shameful, but they can't kill the spirit of Delhi's people," the city's mayor, Arti Mehra, told reporters as she visited one of the blast sites.

Prime Minister Manmohan Singh also condemned the attacks and appealed for calm.

More than 400 people have been killed in bombings across India in the past three years. The attacks usually lead to waves of arrests but are rarely solved.

On July 26, at least 56 people were killed when a series of blasts tore through the western city of Ahmedabad. The blasts occurred outside a diamond market, near a hospital, next to a railway station and in a bus, a day after at least seven small explosions killed two people in Bangalore, a technology hub.

India's Intelligence Bureau received an e-mail from the Indian Mujaheddin about the attacks in Ahmedabad. A similar message was received in May when the pink-walled city of Jaipur, in northwestern India, was rocked by a series of simultaneous blasts that killed more than 83 people and seriously wounded more than 200.