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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 here since the military seized power in a coup two years ago.

It was not clear who was behind the attacks, but police said the violence was not linked to Islamist terrorism. The timing and scope suggested the bombs were set off by opponents of the Southeast Asian nation's ruling junta, which last weekend held a successful referendum on a constitution that critics say will bolster the military's power for years to come.

The first two explosions occurred overnight on a busy street in Hua Hin, around 125 miles south of Bangkok. The tourist city was hit again by another blast on Friday morning.

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Police Lieutenant Chaiyot Thisawong told NBC News that cellphones had been used to detonate the initial devices within a 30-minute period.

An injured tourist is helped after a bomb exploded in Hua Hin, Thailand. DAILYNEWS / AFP - Getty Images

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.

Police said the devices were hidden inside plants on a street filled with restaurants, bars and food vendors that is popular with tourists and local residents.

Two small bombs exploded in the tourist beach town of Patong on Phuket island and two more in Phang Nga, another tourist region north of Phuket on Friday, local police said. One Thai man was lightly wounded in Patong, police said.

Authorities believe the series of blasts were acts of "local sabotage" and not linked to any international militant group, deputy police spokesman Police Colonel Krisana Pattanacharoen told reporters.

Police also said they have not found any evidence that the blasts were related to an insurgency in Muslim-majority provinces in southern Thailand, which has killed more than 6,500 people since 2004.

An investigator collects evidence after the bombing in Hua Hin, Thailand. MUNIR UZ ZAMAN / AFP - Getty Images

However, Hua Hin, Phuket and Phang Nga are far from the usual conflict zone, where attacks typically target security forces and not tourists.

In a separate incident on Friday, Thai media reported two bombs had exploded in the southern province of Surat Thani, killing one person and wounding five.

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

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.