HUA HIN, Thailand — Attackers using firebombs and homemade explosives struck a series of popular resort cities and beach towns across Thailand in some of the worst violence to hit the country since a military coup two years ago. At least four people were killed and dozens wounded, including 11 foreigners.

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

Police said firebombs also triggered blazes at markets and shops in six places, including Phuket, Trang, Surat Thani, Phang Nga and a souvenir shop in the tourist town of Ao Nang, Krabi.

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 visitors rising to 30 million in 2015 and more than 14 million having visited by May 2016, according to the latest official figures.

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

Police said four of the injured tourists were from Germany, two from Italy and one from Austria. The Netherlands said four of its citizens were also wounded.

“It is not yet clear who is behind these attacks,” German Foreign Minister Frank-Walter Steinmeier said in a statement. “But one thing is clear: no political aim justifies violence and attacks on innocent people.”

Thai Prime Minister Prayuth Chan-ocha said the government was doing the best it “could to provide security to the country. … We have to band together and stay strong.”

“Why has it happened now when the country is improving … Who (did it)? Who doesn’t want to see Thailand getting better? Which group? Go find out for me.”

Police said they were investigating all leads and had ruled out links to international militant groups.

Royal Thai Police Col. Krisana Patanacharoen said it was “too early to conclude” who was behind the attacks. But he said the bombings followed “a similar pattern used in the southern parts of the country” — a reference to 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.