Police scour a telephone booth outside the Major Ratchayothin shopping and entertainment centre, the scene of Monday’s late-night bomb blast. The explosion comes as the country prepares to celebrate the Thai New Year from April 13 to April 17.

"We have some information, but it remains incomplete. We are talking with the Metropolitan Police," he said.

A source said after the bomb, security measures were stepped up ahead of the Songkran festival.

Officials are worried about a repeat of the New Year's Eve attacks in the capital and Lunar New Year bombings in the deep South that also killed several. Major Ratchayothin was one of the targets of the December 31 bombs.

Songkran runs officially from April 13 to 17. Hundreds of thousands of revellers are expected on streets across the country.

"Surveillance will focus more on Bangkok than in three South provinces this time," an intelligence source revealed.

Chana Songkhram police superintendent Colonel Utasin Ritruangdej - whose jurisdiction includes Khao San Road - said extra patrols would be deployed in the area over Songkran.

"We have asked the Army Region 1 to deploy soldiers, too," he said. Sniffer dogs would be used around the clock during the festival.

The moves follow orders from Metropolitan Police Commissioner Lt-General Adisorn Nonsee that all capital commanders tighten security, especially at spots where large gatherings were expected.

Adisorn said it was clear the explosion was designed to create a disturbance. "At this point, we have yet to gather enough evidence to determine who is behind the attack."

He believed the bomb was unrelated to the New Year's Eve blasts.

"The bombs used at New Year's were much more powerful," he said.

Metropolitan Police deputy commissioner Maj-General Krissada Phankongchuen said yesterday Monday's device was detonated by a clock and was armed with gunpowder. "It was harmful in a five- to 10-metre radius," he said.

Krissada said the bomb was similar to those detonated outside the residence of Privy Council President Prem Tinsulanonda, Lumpini Park, the Manager Daily building and the Tokyo shopping centre.

National Security Council secretary-general Prakit Prachon-pachanuk and Interior Minister Aree Wongsearaya insisted a state of emergency was unnecessary. Both urged the public to help prevent more attacks.

Police questioned a fruit seller, a traffic-police officer, a traffic-signal technician and a Major Ratcha-yothin security guard after the blast. A Major Ratchayothin executive said the bomb did not stem from a business dispute.