Seven people, including an Italian girl, were injured when a bomb packed in a pickup truck exploded in a shopping mall car park on the popular tourist island of Samui, officials said on Saturday.
The blast occurred about 11.30pm at the Central Festival shopping mall in Surat Thani province. The mall had been open late for holiday shoppers.
The bomb was packed inside a Mazda pickup truck that had been stolen from the southern border province of Yala, police said on Saturday. Pictures of the vehicle identification number and registration documents were posted on the @191Thailand Twitter account.
"It's a car bomb but we cannot confirm what type of explosive materials they used," Lt Gen Prawut Thavornsiri, the national police spokesman, told AFP.
He declined to specify whether police believed the blast was linked to the conflict in the three southern border provinces where a low-level insurgency has killed about 6,300 people over the past 11 years.
Authorities later on Saturday said they believed the incident may have been part of a wider campaign to cause political tension and not directly related to the southern insurgency.
They point to the fact that the blast took place just a half-hour after a fire broke out at the Surat Thani Cooperative on the mainland of the southern province. The Cooperative was founded by politician-turned-monk Suthep Thaugsuban, who led the anti-government protests that culminated in the military coup last year.
Local television reports showed damaged cars and injured shoppers at Central Festival.
"Seven people were slightly injured after being hit by shrapnel," said Pol Col Apichart Boonsriroj, police commander of Surat Thani province on the mainland, adding that they had been released from hospital.
"Six Thais and a 12-year-old girl were treated for minor injuries," said Poonsak Sophonsasmorong of the island's disaster prevention office.
The force of the blast was so heavy that it shook shops on the ground floor and caused heavy damage to goods in some of them.
Both Central Festival and the Sutat Thani Cooperative were closed and cordoned off on Saturday morning.
The southern island of Samui is one of the country's most popular tourist destinations and the explosion occurred at the start of the five-day Songkran holiday, as hundreds of thousands of travellers began converging on vacation spots.
Photographs posted on Twitter on Saturday morning also showed stepped-up security checks on vehicles at the Central Festival mall on Phuket in response to the Samui incident.