A number of Indian media outlets received an e-mail sent just before the blasts Saturday evening warning that India was about to receive "the Message of Death."
"In the name of Allah, Indian Mujahideen strikes back once more. ... Do whatever you can. Stop us if you can," the message said.
All of the bombs exploded in, or very near, crowded shopping areas in various parts of New Delhi. They began going off just before sundown - prime time for weekend shoppers in this crowded, chaotic city.
Home Minister Shivraj Patil said at least 18 people had been killed in five explosions, though some media reports put the death toll as high as 25. Mayor Arti Mehra said at least 61 had been injured.
"It's a very cowardly act of violence," Mehra told reporters near the scene of two of the explosions, in the M-Block market of the city's upscale Greater Kailash neighborhood. "They want to break the spirit of Delhi. They have tried this in other places before and they have not succeeded and they will not succeed here. They will not scare us."
"We will face this difficulty, we will find out who has done it," Patil told reporters.
Joint Commissioner of Police Ajay Kashyap said an unexploded bomb was also recovered near India Gate, a colonial-era memorial that is one of the country's best-known symbols, the Press Trust of India news agency reported. On weekend evenings, the park around India Gate is packed with families who flock there to picnic, enjoy the fountains and buy ice creams for their children.
The Indian Mujahideen was unknown before May, when it claimed responsibility for a series of bombings in the western city of Jaipur that killed 61 people. The group also said it was responsible for July blasts in the western state of Gujarat that killed at least 45.
India, a largely Hindu country, has long battled Muslim militant violence in the Himalayan region of Kashmir, the country's only majority Muslim state, but it was not clear if the Indian Mujahideen was tied to the Kashmiri groups.
U.S. Ambassador David Mulford condemned the attack.
"There is no justification for the vicious murder of innocent people. The U.S. stands shoulder to shoulder with India in the fight against terror," he said in a statement.
The deadliest explosion was believed to be in Gaffar Market in the city's Karol Bagh neighborhood, a warren of stores popular among wholesalers and bargain-hunters. It exploded on a street jammed with clothing shops and stores that sell cheap mobile phones. Hours later, a mangled rickshaw could be seen in front of the small shop where the bomb exploded.
Two of the explosions occurred just 300 yards (meters) apart in Connaught Place, the city's central shopping district. The usually crowded streets quickly emptied of shoppers and filled with screaming police cars, fire engines and gawking crowds.
One of those bombs went off by a subway station entrance on a major road, and a police officer at the scene said it appeared to have been left inside a plastic sidewalk garbage bin that had been ripped apart by the explosion. He declined to be identified because he was not authorized to speak to the media. The sidewalk was covered with garbage, broken glass and a small pool of blood.
Raju Chohan said he was walking through the area with a friend when he heard "a deafening sound and there was some sort of smell in the air."
In the minutes after the blast, the scene was filled with blood and chaos, as police officers raced to the scene and passers-by helped victims into taxis and rickshaws to get to hospitals. A sadhu, a Hindu holy man clad in orange robes, lay face down in the gutter a few feet away, apparently dead. Another man walked away from the scene, helped by other men, his face covered with blood.
A second blast in Connaught Place went off inside a park crowded with families and young people relaxing on the grass.
Raj Kumar, 30, a store clerk in the area, said he was nearby when he heard the explosion.
"Everyone was running every way," he said. "They heard the bomb and they just started running."