HUA HIN, Thailand (AP) — Coordinated explosions targeting some of Thailand's most popular resort cities and beach towns killed at least four people and wounded dozens more. Police said Friday that 10 foreigners were injured in the violence, some of the worst in the country since the military seized power in a coup two years ago.

It was not clear who was behind the attacks Thursday and Friday, which followed a successful referendum held last weekend on a new constitution that critics say will bolster the military's power for years to come.

The explosions all occurred south of Bangkok and several of the blasts — including one on Patong beach in the tourist town of Phuket and three in the seaside resort city of Hua Hin — appeared designed to strike the tourism industry, which provides vital income to the government.

Thailand's economy has sagged since the military seized power in a 2014 coup. But tourism has remained one of the few bright spots, with more than 14 million people visiting through May 2016 compared to nearly 30 million in all of 2015.

Foreign governments, including the United States, issued warnings urging travelers to avoid affected areas.

Police said four of the injured tourists were from Germany, three from the Netherlands, two from Italy and one from Austria.

Police ruled out links to international terrorism or a low-level insurgency in the country's Islamic south that has ground on for more than a decade and killed more than 5,000 people. Militants in the south have carried out such sophisticated attacks before, but almost all have targeted Thailand's three southernmost provinces.

The most devastating explosions occurred overnight in Hua Hin on a busy street filled with bars and restaurants. One Thai woman was killed and about 20 people were wounded, half of them foreigners, according to police Lt Chaiyot Tisawong.

Gen. Sithichai Srisopacharoenrath, the superintendent of police in Hua Hin, said the bombs were hidden inside potted plants and were set off by remote control, half an hour apart. He said a Samsung cell phone had been recovered that they believe was used to detonate at least one the bombs.

On Friday, debris and ball bearings could be seen strewn across the road as police investigated the scene. The blast damaged a pair of phone booths and shattered the window of a nearby Starbucks.

Many shops in the city center closed afterward and normally bustling streets were empty, for good reason: Hua Hin was hit again by another bomb that exploded Friday morning near a clock tower, killing one person and wounding four more.

Separate blasts were reported elsewhere in the south. One exploded on Phuket city's popular Patong beach, injuring one person. Two more detonated in front of two police stations half an hour apart in Surat Thani in southern Thailand, killing one and wounding three. And two bombs exploded outside a market in Phang Na, damaging two vehicles but causing no casualties.

Earlier Thursday, another bomb blew up in the southern province of Trang — full of beautiful beaches and tourist islands — killing one person and injuring six, according to police and Thai press reports.

In Hua Hin, tourist Shane Brett told the Australian Broadcasting Corp. from his hotel room that there was panic after the first explosion there Thursday.

"I was at a bar in the main bar district in Hua Hin right outside the Hilton Hotel and at first I heard kind of a bang ... and everyone kind of panicked," Brett said.

He looked outside and saw people running. Half an hour later, heading back to his hotel, he saw "a good few people injured and the whole area just panicking ... the whole area was just shut down with police cars, ambulances."

Thailand has been plagued by political violence, including several bombings, since the populist billionaire Thaksin Shinawatra was ousted as prime minister in a 2006 military coup after demonstrations accused him of corruption, abuse of power and insulting King Bhumibol Adulyadej.

Thaksin's ouster set off sometimes-bloody battles for power between his supporters and opponents, who include the military. The government of his sister Yingluck Shinawatra, who became prime minister in 2011, was toppled in the country's last coup in 2014.

On Sunday, Thai voters approved a referendum on a new constitution that is supposed to lead to an election next year. Critics say it is undemocratic and is fashioned to keep the military in control for at least five more years even if a free election is held.

In a speech Wednesday night, junta chief and Prime Minister Prayuth Chan-ocha took credit for bringing stability back to Thailand after an extended period of unrest.

On Friday, Prayuth said the government was doing the best it "could to provide security to the country. ... We have to band together and stay strong."

Col. Sansern Kaewkamnerd, a junta spokesman, said Prayuth "expressed his sadness over the unexpected and tragic incident (in Hua Hin)" and ordered police and soldiers in the area to step up security measures.

Friday's blast took place on the birthday of Thailand's Queen Sirikit. The junta has repeatedly said that defending the monarchy is a top priority, and the army and its allies are keen to ensure a smooth succession for Bhumibol, who is the world's longest reigning monarch.

Pitman reported from Bangkok. Associated Press journalists Tassanee Vejpongsa, Grant Peck, Natnicha Chuwiruch and Vicky Ge Huang in Bangkok, and Rod McGuirk in Canberra, Australia, contributed to this report.