Police, military and city leaders stepped up security in the capital yesterday following an explosion just 500 metres from an entrance to Chitralada Palace on Saturday night.

A 22-year-old man received minor injuries in the explosion at a public telephone booth.

Police doubled the number of patrol officers to 3,000 and placed another 1,000 on around-the-clock standby.

The explosion, caused by what police only described as a simple bomb, comes shortly after reports of terror attacks planned for the city this month.

Embassies, government offices and palace compounds are among places to receive extra security.

"We have to be on extra alert and provide additional security measures," police deputy commissioner General Patcharawat Wongsuwan said yesterday after meeting with assistant commissioner Lt-General Jongrak Juthanont, police unit chiefs, officers of the First Army Region and senior Bangkok Metropolitan Administration (BMA) officials.

The BMA will remove little-used telephone booths in remote areas and increase street lighting around others. Jongrak said the administration would also rush the installation of security cameras.

First Army Region Lt-General Prayuth Jan-ocha said the military would work with police to track down Saturday's bomber or bombers.

"We will deploy more officers, especially around palace compounds," he added.

Prayuth said given the fact that only a single bomb, that could have been made by anyone "who went to technical college", was used in the latest bombing, he believed it was the work of "trouble-makers", rather than insurgents or terrorists.

Council for National Security chairman General Sonthi Boonyaratglin yesterday said he believed the latest explosion was aimed at inciting public confusion. But he declined to speculate whether it was politically motivated or the work of terrorists.

Sonthi also dismissed any link to revelations by General Saprang Kalayanamitr that a terror-attack plot for Bangkok had been uncovered in London.

"No, there's no link," Sonthi said yesterday.

A police source said Metropo-litan Police commissioner Lt-General Adisorn Nonsee was commanding a widespread hunt for intelligence.

His deputy Maj-General Viboon Bangthamai said the Army and national and city police would work together. "We will set up 12 new checkpoints and search persons and motorists," he said.

He said around-the-clock checkpoints would be concentrated at palaces.

Patcharawat explained Saturday's bomb was similar to one exploded near the Major Cineplex Ratchayothin last month. Police have questioned four witnesses to the latest incident.

The bomb was designed to create fear not kill, he said. "No expertise is required to produce such a bomb," Patcharawat said.

He added it was too early to say if the incident was politically motivated.

In its wake Bangkok Governor Apirak Kosayodhin recalled several district chiefs from overseas.

"The situation is not normal. It's best they are here and ready to respond," Apirak said.

District office directors will ensure around-the-clock patrols, efficient in formation gathering and the preparedness of rescue and safety equipment.

Metropolitan Internal Security Operations Command deputy director Colonel Dithphop Kaikaew warned targets could shift from property to persons.

"District office directors must be vigilant and take good care of themselves, too," he said.