Yesterday's pre-dawn explosion at the Customs Department headquarters near Bangkok Port caused only minor property damage, police said.
In another incident in Bueng Kum district, two grenades of the type fired from an M203 launcher were found near Pattawikorn Market.
Meanwhile, in Phayao a bomb and AK-47 assault rifle attack on Bangkok Bank's Dok Kham Tai branch caused damage estimated at Bt300,000.
Following a report of a bombing at the Customs Department around 3am, police found the windows and part of the ceiling of the 120th Anniversary Building as well as a van parked in front were damaged.
The blast caused a 10-centimetre hole in the yard. No deaths or injuries were reported.
Maj-General Boonsong Phanich-attra, deputy Metropolitan Police chief, who led a bomb team to inspect the scene, said someone must have taken the K75 hand grenade from the roadside to the compound.
He declined to say whether the attack had anything to do with the political situation, with red-shirt protesters rallying in Bangkok for Prime Minister Abhisit Vejjajiva to dissolve the House.
Pol Colonel Sombat Milinthajinda said security camera footage showed two men on a motorcycle launching the attack, but the image needed technical sharpening so the men and the bike model could be clearly identified.
The motive could be either anger towards Customs officials regarding products held on probation or the intention to create chaos, he added.
The two grenades discovered about 100 metres from Pattawikorn Market at 6.30am were believed to be intended for an attack, but abandoned to avoid a checkpoint nearby. While the information on one was still intact, the serial number and manufacturing data on the other were scraped off.
In the North, no one was killed or wounded in the attack on the Bangkok Bank branch at 2am, but police found 23 spent AK-47 cartridges along with some bomb fragments in front. Pol Lt-Colonel Kiattipong Khampinchai said the attack was suspected to be an attempt by an ill-intentioned group to create chaos rather than to harm bank workers or property.
Branch manager Manas Pinjai said the bank had no conflicts with the community or any individual or former employee who might hold a grudge against the bank. The damage was about Bt300,000, he added.
