Security guards alerted Makkasan police at around 2am to investigate a loud noise heard near the entrance to the building on New Phetchaburi Road.

They told police they had entered the building at about 1am to seek shelter from heavy rain. Later they spotted two unidentified men on a motorcycle throwing a small object at the gate.

They said the two men had returned about 40 minutes later and thrown an object that sounded an explosive. The incident caused no injuries or damage.

Major Rungphet Makee, duty officer at Makkasan Police Station, said the fake grenades had been assembled in plastic shells to look like M-60 grenades.

Metropolitan Police Region 1 commander Maj-General Pramote Pathumwong said the bomb squad had confirmed that the fake grenades were used for special effects and contained no explosives.

"Police have not drawn any conclusions, although they are quite certain that the perpetrators wanted to create a disturbance rather than inflict any serious damage," he said.

A team of 10 policemen has been dispatched to guard the Thai Rak Thai headquarters as a precaution, he said. Party leader Thaksin Shinawatra said police were trying to solve the case. He refused to comment on the incident, saying he wanted to spend time with his family on a Sunday.

Party executive Suranand Vejjajiva said police should quickly identify the culprits in order to clear up a murky political situation.

"Thai Rak Thai fully embraces democratic rule, and its opponents should play by the rules instead of resorting to intimidation," he said.