An undisclosed number of Red Shirt supporters of the Pheu Thai party have been detained in the country's southern provinces and taken to a military base near Bangkok for questioning.

The country's military rulers have sweeping powers to arrest and interrogate anyone.

Thai police have said that one mastermind was behind the wave of attacks on popular tourist destinations that killed four people and wounded dozens.

Deputy National Police Chief Pongsapat Pongcharoen said police have a "clear idea" of who was behind the bomb attacks.

"The events are connected, carefully planned and carried out across many areas and masterminded by one individual," he said, without elaborating on who the individual was or their motive.

Noppadon Pattama, a key Pheu Thai figure, told reporters that exiled former prime minister Thaksin Shinawatra had instructed his lawyers to file police complaints against those accusing him of orchestrating the blasts that killed four people and injured 33 others, 11 of them foreigners.

"People are sending messages through social media accusing Thaksin Shinawatra," Mr Noppadon said, referring to tens of thousands of posts on social media sites.

"This is slander and defamation. All former prime ministers love and worry for their country. None of them would condone such evil acts."

Thailand has been divided for a decade by populist political forces led by Thaksin, who was toppled in a 2006 coup, and the royalist and military establishment, which accuses him of corruption.

Thaksin's younger sister Yingluck Shinawatra, who was prime minister when Pheu Thai was forced from office in 2014, has also condemned the bombings as inhumane.

Authorities have increased security in resorts and across Bangkok after the attacks that targeted areas popular with foreign tourists, including Phuket, where up to 25,000 Australians holiday each month.

Australia has warned its citizens in Thailand to avoid crowds and remain vigilant.

No group has claimed responsibility for the attacks, the first violent challenge to the military that topped the Pheu Thai government after months of street unrest in May 2014.

The military government, worried about the bombings damaging tourism that accounts for more than 10 per cent of an already sluggish economy, was quick to rule out terrorism.

Authorities said evidence points to disgruntled political parties but he government has not accused Pheu Thai or Mr Thaksin, a divisive figure whose parties have won the last five elections.

Pro-coup Democrat Party politicians have also denied the attacks were linked to a conflict within their ranks over a referendum on August 7, in which voters approved a military-backed constitution that weakens the power of political parties and perpetuates the military's key role in government for the foreseeable future.

The military also dismissed a rumour that soldiers were behind the blasts.

"The military will never harm the people. I can vouch for that," said Colonel Piyapong Klinphan, a spokesman for the National Council for Peace and Order.

A total of 16 explosive devices have been found in the country since August 10 as well as a series of arson attacks.

The latest two devices were found in a market area in southern Phang Nga on Saturday, in an area where arsonists lit a fire on Friday.

Investigators say some of the bombs were detonated by telephone sim cards from Malaysia.

The bombs were similar to those used by Muslim separatist insurgents during the decade-long southern conflict which has claimed more than 6000 lives since 2004.

Police said DNA samples gathered at bomb sites would be compared with DNA samples stored in databases in the south.

Security experts say the bombs are not difficult to construct, with instructions available on the internet.

Two bombs that exploded in the resort town of Hua Hin on Thursday night were packed with shrapnel but others, like those detonated in Phuket on Friday morning, appeared to be designed not to inflict mass casualties.

with DPA