A series of blasts hit three of the most popular tourist resorts as well as towns in southern Thailand on Thursday and Friday, killing four people and wounding dozens, days after the country voted to accept a military-backed constitution.

Four bombs exploded in the upscale resort of Hua Hin, about 200 km (125 miles) south of Bangkok, on Thursday evening and Friday morning, killing two people and wounding at least 24.

Other blasts hit the tourist island of Phuket, a resort town in Phang Nga province, and Surat Thani, a city that is the gateway to popular islands such as Koh Samui in Thailand's Gulf.

Hua Hin is home to the Klai Kangwon royal palace, which translates as "Far from Worries", where King Bhumibol Adulayadej, the world's longest reigning monarch, and his wife, Queen Sirikit, have lived in recent years. Neither were there during the attacks, as both are in hospital in Bangkok.

Friday was a public holiday in Thailand to mark the queen's birthday, which is celebrated as Mother's Day.

No group has claimed responsibility, though suspicion could fall on insurgents in Muslim-majority provinces in southern Thailand.

ATTACKS IN SEVEN PROVINCES

Police detained two men for questioning over the attacks on Hua Hin on Friday, said Police Superintendent Sarawut Tankul of the tourist police at the resort. They were detained because CCTV footage showed them in the area "before, during and after the bombings", he said, declining to give more details.

Preliminary evidence showed the bombs were low-explosive devices devised to "make an announcement" rather than cause maximum harm, he said.

Ahead of the blasts, Police had intelligence an attack was imminent, but had no precise information on location or timing, national police chief Chakthip Chaijinda told reporters in Bangkok on Friday.

"We just didn't know which day something would happen," he said.

Since Sunday's referendum on the constitution, there have been attacks in seven provinces using improvised explosive devices and firebombs, Chakthip said.

The devices were similar to those used by separatist groups in southern Thailand, but that did not conclusively show they were the perpetrators, he said.

Police ruled out any links to international terrorism, as did Thailand's Foreign Ministry, which said in a statement on Friday: "The incident is not linked to terrorism but is an act of stirring up public disturbance."

Bangkok-based analyst Anthony Davis, at security consulting firm IHS-Jane's, questioned the police assertion that the attacks were an act of local sabotage.

The coordinated bombings were "designed to terrify and to blow a hole in the tourism industry", said Davis.

"The tactics used were clearly intended to minimise casualties while maximising economic and political impact. It is worth noting that no foreigners died in the attacks."

Thai authorities beefed up security at tourism spots, airports and on public transport in Bangkok.

"Why now, when the country is getting better, the economy is getting better, and tourism is getting better? We have to ask why and who did it," Thai junta chief and Prime Minister Prayuth Chan-ocha told reporters.

TRAVEL ADVISORY

The attacks are bad news for Thailand's tourist sector, which has been one of the few bright spots in a sluggish economy.

Tourism accounts for about 10 percent of gross domestic product and Thailand was expecting a record 32 million visitors this year.

Australia issued a travel advisory saying Australians should "exercise a high degree of caution" and warned: "Further explosions in any part of Thailand are possible."

Two blasts on Friday morning in Hua Hin came after twin explosions late on Thursday. The explosion that wreaked the most damage was near a bar in a bustling narrow alley in the town late on Thursday. It killed one Thai woman and wounded 21 people, Krisana said.

The streets were spattered with blood and debris on Friday in front of the bar where the explosion took place.

The blast peppered the bar with shrapnel and carpeted the road with those too badly wounded to flee, said Chayanin Seedee, 26, who manages the premises.

"Right now, we're just very scared," she said.

Ten of those wounded in the Hua Hin blasts were foreigners, Krisana said, and eight of them were women.

Such twin blasts are common in the three Muslim-majority southernmost provinces of Thailand, where a long-running insurgency intensified in 2004, with more than 6,500 people killed since then.

The three provinces, near the border with Malaysia, soundly rejected the referendum on the new military-backed constitution, which passed convincingly in most of the rest of the country in Sunday's vote.

Violence has occasionally spilled over to areas outside the three provinces, which were part of a Malay sultanate until they were annexed by Buddhist-majority Thailand a century ago.

Hua Hin, Phuket and Phang Nga are far from the usual conflict zone, where attacks are typically aimed at the security forces and government representatives, not tourists.

In a separate incident on Friday, media reported two bombs had exploded in the southern province of Surat Thani, killing one person and wounding five. That came after a blast in Trang, also in the south, on Thursday, in which one person died and six were wounded.

No one was killed or seriously wounded on Friday in two blasts in the beach town of Patong, on Phuket island, or the two explosions in the beach province of Phang Nga. Authorities also defused two explosive devices in Phuket on Wednesday, police said.

The head of Interpol in Thailand, Police Major General Apichat Suriboonya, told Reuters it appears the bombs were meant more to send a message rather than cause death and destruction.

"But the thing is, if you observe the bombs, they are not targeted to kill people but to send a message to some groups," he said.

Small bombs have been used frequently for attacks during periods of unrest over the past decade of political turmoil, but have been rare since the military seized power in a 2014 coup.

The latest bombings came almost a year after an attack on a Hindu shrine, crowded with tourists in central Bangkok, killed 20 people and wounded more than 120. Police have accused two ethnic Uighur Muslims from China for the Aug. 17, 2015, attack. – Reuters