Police are continuing their search for the suspects behind a series of bomb blasts across Thailand that killed four people and and injured more than 30, including tourists.
The country was on edge after 11 small bombs exploded across five southern provinces on Thursday night and Friday morning.
The bombs, most of them detonated in double blasts, struck key tourism hubs during a long weekend.
Two bombs exploded in the upmarket resort of Hua Hin late on Thursday, killing one woman and wounding more than 20 others.
They were followed by two more on Friday morning that killed another person.
A further two blasts struck on Friday in the popular tourist town of Phuket, while two more bombs were reported in the southern provinces of Trang and Surat Thani, in each of which one person was killed.
"This incident will remind all Thais that there are still ill-intended people in the country," Prayuth Chan-o-cha, Thailand's prime minister, said in a televised speech on Friday.
"I would like to urge all Thais and the media not to panic. From now on, please help keep an eye on the situation, and arrange volunteers in your area to monitor all important places as well as tourist attractions in every province."
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No one has claimed responsibility for the attacks, but police have ruled out international terrorism and said the campaign was an act of "local sabotage".
"There have been no arrests yet," Piyapan Pingmuang, deputy national police spokesperson, told AFP news agency on Saturday morning.
He said police have yet to identify any suspects or a motive.
Speaking to Al Jazeera from Hua Hin on Friday, Thitinan Pongsudhirak, director of the Institute for Security and International Studies at Chulalongkorn University, said: "I think the timing is striking here. Today is the 84th anniversary of the birthday of Her Majesty [Queen Sirikit].
"We had the referendum last Sunday. So it seems clear to me that this is a coordinated round of bomb attacks.
"I think this has to do with domestic politics. It has something to do with anti-regime sentiments - anti-regime people who want to send a message that they don't like the outcome of the referendum."
|The prime minister said he did not know who was behind the attacks [Yongyot Pruksarak/EPA]
Chakthip Chaijinda, the national police chief, said in Bangkok on Friday that authorities had received intelligence that an attack was imminent, but police had no precise information on location or timing.
"We just didn't know which day something would happen," he said.
Authorities have also been quick to play down possible links to a separatist movement in Thailand's southern border region.
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The explosive devices were similar to those used by separatists in southern Thailand, but that did not conclusively show they were the perpetrators, Chakthip said.
If the Muslim rebels in the south are to blame, it would mark a major expansion of their secessionist campaign that has been concentrated further south and rarely targets foreigners.
Last week, Thailand voted to accept a military-backed constitution despite claims by opponents that it will entrench the military's power and deepen divisions.
Famed for its idyllic islands and Buddhist temples, Thailand is a tourism powerhouse and was hoping for a record 32 million visitors in 2016.
The industry accounts for at least 10 percent of a lagging economy that the military government, which came to power in 2014, has struggled to invigorate.
Thailand's reputation as a holidaymaker's paradise has in recent years weathered bus and boat accidents, bouts of political unrest and crimes against foreigners.
|Famed for its idyllic islands, Thailand is a tourism powerhouse [Athit Perawongmetha/Reuters]
Embassies in Thailand warned their citizens to stay vigilant on Friday and some warned that there could be more attacks.
Australia, the source of just over 800,000 visitors to Thailand in 2015, issued a travel advisory saying Australians should "exercise a high degree of caution", and warned that "further explosions in any part of Thailand are possible."
China, the single biggest source of tourists to Thailand with nearly eight million visitors in 2015, told citizens to avoid crowded areas and pay close attention to security developments.
Tour operators TUI and Thomas Cook said customers booked on trips to Thailand departing before August 15 could cancel or rebook to other destinations free of charge.
TUI, which has about 2,000 customers in Thailand at present, said it would help any who wished to leave early.
Kobkarn Wattanavrangkul, Thai tourism minister, said the sector was resilient and would rebound from any impact fast, as it had in the past.
A bomb attack at a Hindu shrine located among luxury shopping malls and hotels in central Bangkok killed 20 people and injured more than 120 others in August 2015.
Tourist arrivals dropped by more than half a million in the month after the blast, but Thailand still hosted a record 29.88 million visitors in 2015.
A year earlier, months of sometimes deadly street protests paralysed Bangkok before a coup in May 2014.
The month after the military takeover, tourist numbers slipped to their lowest in more than two and a half years.
Source: Al Jazeera and agencies