A series of deadly bombings across central and southern Thailand that left four people dead and more than 30 injured was likely triggered by the political situation in the country, Thai police said Friday.
Police suspect that "local elements" carried out the attacks, which struck tourist destinations and official buildings across five provinces.
The four dead were all Thai, the government said.
The most serious blast, which struck around midnight (1700 GMT Thursday) in the busy seaside resort of Hua Hin, killed one person and injured 20 more. Those injured included three Germans, three Italians, three Dutch citizens and one Austrian, according to official tallies.
Speaking from the National Police Headquarters in Bangkok, Police Chief Chakthip Chaijinda told reporters that he believed they were carried out by people dissatisfied with the political situation in Thailand, which is currently in its third year under a military government.
Chakthip said it could have been carried out by opponents of a military-drafted constitution, which was approved in a referendum last week.
"You have to look at where the attacks occurred, all these provinces voted for [the military constitution]," he said. "This is just my opinion, but these attacks were in relation to the political developments."
Foreign embassies, including the US, German and British authorities, expressed their sadness at the bombings and urged their citizens exercise caution if they were in Thailand, one of Asia's top tourist destinations.
Foreign tourist arrivals in Thailand have doubled in the last 5 years to 30 million annually, according to the World Tourism Organization.
Security was tightened in all provinces after the overnight and early morning attacks, reports said. Police dogs and soldiers were seen patrolling railway stations and government installations in the capital of Bangkok.
Nittinai Sirismatthakarn, president of Airports of Thailand, told dpa that security at the country's airports was increased to its highest level.
The junta had urged greater vigilance ahead of Thai Queen Sirikit's birthday celebrations on Friday, spokesman Winthai Suvaree said.
"We will not let people with bad intentions towards the country dictate the situation," Winthai told reporters.
The attacks were not "terrorism-related," Thai police said.
"Thailand has no territorial or religious quarrel with anyone. This is not an act of terrorism but an act of local sabotage," police spokesman Piyaphan Phingmuang said.
One person was killed and three injured in a second bomb attack in Hua Hin around 9 am.
The earliest blast was on Thursday evening in the southern province of Trang, 850 kilometres south of Bangkok. Two explosions targeted police stations in the southern province of Surat Thani around 8 am on Friday, killing one.
Another bomb hit the tourist resort of Phuket around 9 am, injuring a taxi driver. A bomb blast targeted a tourist area in southern Phang Nga province, but no one was injured.
"The bombings have the same attack profile," Army General Danai Kritmethavee told reporters. "We believe at this time that this was a coordinated attack."
"These attacks are aimed at creating unrest in the country," Deputy Prime Minister Prawit Wongsuwan told reporters Friday morning.
Prime Minister Prayut Chan-ocha said "we have to focus on not creating panic among the people and restoring law and order."
Authorities provided little evidence of who was behind the attacks, which come as Thais marked a long weekend to celebrate Queen Sirikit's 84th birthday, also Mother's Day in Thailand.
Many Thais either return home or spend the extended break at seaside resorts like Hua Hin.
Thailand sees regular bomb attacks, but rarely beyond its most southern areas. Since 2004, an insurgent movement to create an independent state in three southern Muslim-majority provinces has resulted in 6,200 deaths and 11,000 people injured.
In five days' time the country will mark one year since one of its worst bomb attacks. The bombing of the Erawan shrine on August 17 last year killed 20 people, mostly tourists.