The explosion took place near the stage inside the grounds of the prime minister's offices, where supporters of the People's Alliance for Democracy (PAD) have been camped out since August.

'We don't know what kind of bomb it was because as usual they (PAD) don't allow police in immediately to examine the site inside Government House,' police officer Colonel Somchai Chueyklin said.

The bomb comes after another explosion at the same site last Saturday left three protesters injured, according to the PAD.

A day later two PAD volunteer guards were arrested and have since been charged with drugs and weapons offences.

The pair, who PAD leaders say are no longer part of the group, were found with 23 homemade bombs, three grenades, four bullets, knives and sticks, according to local media.

PAD accuses the government of corruption and took their protest to parliament on October 7, causing riot police to fire tear gas that led to clashes leaving two dead and nearly 500 injured. -- AFP