A bomb exploded outside a horse stable near the Thai prime minister's residence in Bangkok early on Sunday, but no one was injured, local press reports said.
The blast, which followed two earlier explosions on Friday that injured three people, raised political tension and prompted police to beef up security in the capital and surrounding areas.
The Bangkok Post website quoted police as saying they found urea fertilizer, diesel fuel and metal fragments at the scene of Sunday's explosion at Nang Loeng Horse Stable, near Phitsanulok House, the prime minister's official residence.
Metropolitan Police Division 1 Deputy Commander Weerawit Chanchamroen was quoted as saying that the suspected home-made bomb went off about 12.30 am and damaged a nearby stadium wall and a bridge.
He said police were checking to see if the bomber had been filmed on one of the closed-circuit television cameras installed near the site of the explosion in the city's government quarter.
Bangkok has been plagued by a series of bombings near government buildings and companies affiliated with the government since May 19, when army troops broke up a mass protest in central Bangkok. A total of 91 people died and over 1,000 were injured in clashes between troops and police and anti-government protesters in April and May.
The government of Prime Minister Abhisit Vejjajiva has cited the bombings as justification for maintaining a state of emergency in Bangkok and six other provinces.