Hundreds of protesters trapped Prime Minister Abhisit Vejjajiva inside a beach hotel for several hours on Tuesday, raising tensions ahead of a key Asian summit beginning there on Friday.
They later attacked his motorcade when he left.
The clash was the most serious since supporters of fugitive former prime minister Thaksin Shinawatra surrounded Government House in Bangkok on March 26, and came amid mounting speculation of a possible military coup.
Red-clad Thaksin supporters calling for fresh elections surrounded Abhisit's hotel in Pattaya as he held a cabinet meeting, preventing him from leaving for several hours, witnesses said.
Abhisit finally escaped but about red-shirts chased after his car when it stopped at a traffic light and then pelted it with plastic bottles, police who saw the incident told AFP.
Police tried to move the prime minister to another car but the demonstrators pulled open the door of the second vehicle and began to beat the driver and police who were escorting Mr Abhisit, they said.
The Oxford-educated Abhisit escaped unharmed, they added.
Deputy Prime Minister Suthep Thaugsuban, who is in charge of security, said he had ordered police to catch the attackers immediately and added that police had photo evidence of the incident.
"I told police to arrest the assailants within today," he told reporters. "This is not a peaceful protest. They have violated the law, the government already warned them that they will be prosecuted."
Pattaya is set to host leaders of the Association of Southeast Asian Nations (Asean) and key regional partners including China and Japan at a summit starting on Friday.
The summit -- already postponed from December due to ongoing political unrest -- would go ahead as planned, said Vitawas Srivihok, the senior Asean department official at Thailand's foreign ministry.
"The red-shirts protest will not affect foreign leaders attending the summit," Mr Vitawas said.
The incident came a day before a mass rally planned by Thaksin's supporters in Bangkok which organisers say hundreds of thousands of people are expected to attend.
Thailand, a key tourism destination, has been crippled by political turmoil since billionaire tycoon Thaksin was toppled in a military coup in September 2006.
He is living in exile to avoid a two-year jail term imposed last year after a court convicted him of corruption in office. He claims the case was politically motivated.
Source: Bangkok Post