The Delhi [Images] police on Saturday sounded an alert for two men who dropped the bomb in the crowded Mehrauli market and launched extensive checking across the capital.

The two men, believed to be aged around 30-32 years, riding a black motorcycle with a Delhi registration number were responsible for the explosion in which a boy was killed and 18 other people injured.

"We have sounded an alert for them. Extensive checking is being undertaken to nab them," Joint Commissioner of Police (Southern Range) Ajay Kashyap said.

Considering that Mehrauli is located on the border with Haryana, checking of vehicles has been intensified at the Delhi-Gurgaon border.

The two well-built suspects were wearing helmets and black jeans and black T-shirts when they carried out the blast.

The person riding in pillion dropped the packet, containing a tiffin-box in which the explosive was hidden, near an electronic shop in Mehrauli market.

A child picked up the packet and wanted to return it to the motorcyle riders, but it exploded killing him.

Three persons, including a couple who saw the entire episode, are being questioned.

"The two bombers were in black dress, were wearing black helmets and riding a black Bajaj Pulsar. So it has not been possible for the eyewitnesses to identify them," Kashyap told reporters hours after the blast.

Kashyap said the Delhi police was in touch with the Gurgaon police as the blast site was located near the city.

"We have also issued a look out notice," he said.