Phuket police have urged calm after about 1,000 people descended on a soybean milk shop and accused the owner's son of defaming the late King on Facebook.
The demonstrators streamed into Yaowarat Road in tambon Talat Yai in Muang district at about 11.30pm on Thursday, calling on police to press charges.
Their dissatisfaction looked set to escalate when police and military officers, led by Muang Police chief Pol Col Kamon Osiri, failed to talk them out of the protest and they set up a roadblock.
The protesters raised doubt about police handling of the case after officers earlier questioned the son and released him without charge.
Phuket police chief Pol Maj Gen Thiraphon explained police had no power to detain the son, whose name was not revealed, because no wrongdoing had been seen by officers and the words posted online were not considered "direct defamation".
He said if officers gathered sufficient evidence, they would ask a court for an arrest warrant.
However, his clarification failed to satisfy the protesters and at 12.30am he asked them to open the way to traffic.
The situation began to calm down when Suthin Lianudom, a former mayor of Ratsada municipality, said he would act on behalf of the demonstrators in lodging a complaint with Muang Phuket police.
Mr Suthin was a leader of the People's Democratic Reform Committee in Phuket, which joined the 2014 protest movement against the Yingluck administration.
Mr Suthin then accused the son of violating Section 112 of the Criminal Code, known as the lese majeste law, and the Computer Crime Act.
Pol Maj Gen Thiraphon said he understood how the protesters felt and promised a speedy investigation.
The Thai people are dealing with a great loss, so everyone must unite and "let laws be above emotions", he said.
