The Thai capital Bangkok is on alert for possible bomb attacks against some 100 targets. Local media reported that police received information of plots from two men who were arrested on Saturday after setting off a grenade outside a criminal court.
They implicated three others and claimed that red-shirt activists were behind the plots. The red-shirts are supporters of the administration of former prime minister Yingluck Shinawatra.
One of the detained men allegedly told police he was paid to carry out the bombing on the court. He also said it was part of a plot to stage violent acts in order to draw the attention of the United Nations to Thailand.
On Saturday, a small bomb detonated outside a court building in central Bangkok, but the blast caused no casualties. The explosion occurred in a car park outside the Criminal Court on Ratchadaphisek Road, one of the main thoroughfares running through Thailand's capital.
The latest blast comes just over a month after twin pipe bombs exploded outside a downtown shopping mall slightly injuring two people in what was the first major disruption to an uneasy peace imposed under martial law since last May's coup. Authorities say they are looking for two men seen on CCTV shortly before those bombs detonated.
The explosions come at a time of heightened political tension in the deeply divided nation. Thailand has been under martial law since the coup toppled the elected government last year, despite growing calls from rights groups and the international community for the junta to rescind the draconian law.
Political gatherings and criticism of the coup are forbidden under the law, with the junta responding aggressively to any form of protest. Martial law also allows civilians to be tried in military courts, with verdicts not subject to appeal.
The Thai junta has repeatedly justified martial law by saying Thailand's bitter political divisions need more time to heal, while they set about remodelling the kingdom's political structure.
Thailand has endured nearly a decade of political turmoil since billionaire former premier Thaksin Shinawatra rattled the establishment by winning elections with his populist politics. Parties led by Thaksin, his family or affiliates have won every election since 2001, in the process facing two coups and the disposal of three premiers by Thailand's interventionist courts.