HUA HIN, Thailand » A wave of coordinated explosions rattled multiple cities across southern Thailand, killing at least four people and wounding dozens more, police said Friday. Among the injured were 10 foreigners in the seaside resort town of Hua Hin.

It was not clear who was behind the attacks, but police said the violence was not linked to Islamic terrorism. The timing and scope suggested the bombs were set off by opponents of the Southeast Asian nation’s ruling junta, which last weekend organized a successful referendum on a 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 resort of Phuket — appeared designed to hit the tourism industry.

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 in 2016 so far — up from 12.5 million the year before.

The first two explosions occurred overnight on a busy street in the tourist city of Hua Hin, which was hit again by another blast on Friday morning. The city is home to a swath of beachfront resorts as well as a royal palace.

Police and Thai media reported other blasts the southern cities of Phuket, Trang and Surat Thani.

Royal Thai Police official Col. Krisana Patanacharoen said it was too soon say who was behind the attacks, but “we are sure that it is not linked to terrorism.”

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 ailing 88-year-old King Bhumibol Adulyadej, who is the world’s longest reigning monarch.

Tourist Shane Brett told the Australian Broadcasting Corp. from his hotel room in Hua Hin that there was panic after the first explosion, which police said killed one Thai woman and wounded about 20 others, half of them foreigners.

“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 the bar and said saw people running. Half an hour later, he made it back to his hotel. On the way, he said he saw “a good few people injured and the whole area just panicking … the whole area was just shut down with police cars, ambulances.”

The bombs Thursday were set off by remote control, half an hour apart, according to Pol. Gen. Sithichai Srisopacharoenrath, the superintendent of police in Hua Hin, and his deputy, Lt. Gen. Samer Yousamran.

Sithichai said both devices were hidden inside plants on a street filled with restaurants, bars and food vendors that is popular with tourists and local residents. He said a Samsung cell phone had been recovered that they believe was used to detonate at least one the bombs.

The fatality Thursday was described in Thai media as a female street food vendor. Several of the injured were in serious condition, the reports said. Pol. Lt Chaiyot Tisawong, an officer in Hua Hin, said 10 of the injured were foreigners. Their nationalities were not immediately known.

On Friday morning, two more bombs exploded in Hua Hin, killing one person and wounding four, according to police.

Another pair of bombs exploded in front of two police stations half an hour apart in Surat Thani in southern Thailand.

Earlier Thursday, another bomb blew up in the southern province of Trang, killing one person and injuring six, according to Thai press reports.

Trang is on the fringes of Thailand’s deep south, where a low-level Muslim separatist insurgency had killed more than 5,000 people since 2004. Almost all the violence has been in the three southernmost provinces.

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.

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 ousted 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.

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