Riding a black motorcycle, the two men in black dress and wearing helmets, dropped a black polythene bag containing a tiffin box opposite an electronic goods shop in the flower market in Mehrauli at about 2.15 pm, Deputy Commissioner of Police (South) H S Dhaliwal said.

A 13-year-old boy picked it up to give it back to the bikers. They refused and fled the place but the boy was still keen on giving it to them when it exploded killing him instantaneously and injuring 18 others.

Eleven of the injured were rushed to AIIMS where doctors said six were in critical condition. Five of them suffered serious head injuries. Three or four of the injured could be required to be operated upon, Union Home Secretary Madhukar Gupta said.

The explosive shattered the window panes of several adjacent shops in the market located close to the historic Qutab Minar, which was bustling with shoppers.

The blast, which caused a crater, left a gory scene where blood was splattered on the road along with shattered glass panes and furniture.

The explosion comes exactly two weeks after the national capital was rocked by five near simultaneous blasts in which 24 people were killed.

There were reports that police in the neighbouring Faridabad district in Haryana had tipped off about the possibility of explosions on Saturday but Gupta asked the media not to speculate.

Soon after the explosion, Home Minister Shivraj Patil reviewed the situation with top officials of his Ministry.

Gupta, who talked to Delhi Police Commissioner Y S Dadwal, briefed Patil on the situation.

The nature of the explosives used in Saturday's blast was not immediately known. Forensic experts of the NSG bomb data squad, who visited the spot, found the nails in the explosives were sharpened on both sides.

Immediately after the explosion, shopkeepers and shoppers rushed to help the injured people and took them to nearby hospitals.

"There was a huge sound. I came out of my shop and saw some people were injured and screaming for help," a shopkeeper said.

People in the area displayed their anger by alleging that police came late despite being called several times.

"Police were nowhere to be seen till one hour after the incident. We took the injured to the hospital," a shopkeeper, who was injured in the incident, said.

Police have cordoned off the area and senior police officials have rushed to the spot to take stock of the situation.