Authorities say evidence is pointing to disgruntled political parties affected by the coup for the bombings in seven provinces, as two new unexploded devices were found in Phangnga province on Saturday
The National Council for Peace and Order said its assessment was that political parties were involved, and ruled out the southern insurgency spreading wider or foreign terrorism as causes.
As tens of thousands of comments across social media accused the Pheu Thai party of being involved, key party figure Noppadon Pattama issued denials and threats of lawsuits.
Mr Noppadon, who expressed condolences to the families of the victims, urged critics not to jump to conclusions that the violence was politically motivated. He threatened to sue anyone who linked former prime minister Thaksin Shinawatra to the violence.
"People through social media are sending messages saying Thaksin Shinawatra is behind these events," Mr Noppadon said.
"This is slander and defamation. Any former prime minister would be worried about their country and would not do such evil."
NCPO spokesman Piyapong Klinpan said one suspect has been arrested and six activists detained after the blasts which killed four people and injured 35.
The 13 bomb blasts and four arson attacks hit Trang, Hua Hin in Prachuap Khiri Khan, Phuket, Surat Thani, Phangnga, Krabi and Nakhon Si Thammarat on Thursday and Friday.
The violence occurred less than a week after the draft charter, which critics say is intended to strengthen the power of the military and prolong its hold on power, was passed.
Deputy national police chief Pongsapat Pongcharoen said while the explosive devices were similar to those used in the South, the attacks are not related to the insurgency.
He said it was highly likely they were carried out by interests working for a mastermind.
A court in Nakhon Si Thammarat has issued a warrant for the arrest of a man in connection with one of the attacks in the province, which involved a Tesco hyper-market, a source close to the investigation said.
The source said police reviewed security camera footage and identified the suspect, but have revealed no details about him.
About 20 people have been questioned and three damaged mobile phones of the same model used in the attack seized for examination.
Earlier, a man identified as Sakarin Karuehas, 32, a Chiang Mai native, was detained on an oil rig platform in the Gulf of Thailand and taken to military premises in Nakhon Si Thammarat for questioning.
Pol Maj Gen Wanchai Ekpornpit, the Nakhon Si Thammarat police chief, said authorities have clear evidence indicating Mr Sakarin was involved with the Tesco arson.
One of the six political activists detained by the military is identified as 67-year-old Prapas Rojanapithak, a native of Trang.
Acting under Section 44 order of the interim constitution, which gives the military power to summon, arrest or detain suspects in a range of security-related cases, troops from the Fourth Army Region detained him at his house in Trang's Muang district and took him to an army camp in Nakhon Si Thammarat.
They confiscated books and documents from his house for examination. He was detained on suspicion that he was linked to the bombings in the south.
Mr Prapas denied any involvement and said he had no links to red shirts or the United Front for Democracy against Dictatorship (UDD).
He admitted he had exchanged political views with friends who shared the same ideology, but said he had never taken part in any political movement. In Phangnga's Takuapa district yesterday, security officials found two bombs in front of a market. One of them was defused by an EOD team while the other was found to be a dud. Both devices were to have been detonated by mobile phones of the same model used in the other attacks.
The devices were sent to Khao Lak police station and will be examined for fingerprints.
Deputy national police chief Srivara Ransibrahmanakul has been appointed to head the investigation into the bombings and arson attacks.
Lt Gen Nanthadej Meksawat, a former official at the Armed Forces Security Centre, said those responsible for the bombs wanted to mislead the public into thinking the violence is related to the insurgency.
He said the attackers also picked tourist provinces because security in those places was not especially tight and the explosive devices were not complicated to construct.
Moreover, the targets were provinces where approval of the draft charter was high.
"I don't think it's insurgents ... the likely suspect is a network of politicians who are against the government," he said.