Swimmers fled the water at normally clean Hua Hin beach in Prachuap Khiri Khan province on Tuesday as oil and oil coated garbage washed ashore there for the first time.
Hua Hin mayor Nopporn Wuthikul then closed the beach and ordered local health and environmental officials to clean it up and collect samples for testing.
Hua Hin deputy mayor Pailin Kongphan said it was the first time oil had washed up on Hua Hin beach.
The navy had been asked to track down the source of the oil slick, he said.
Large quantities of greasy garbage and balls of oil blackened sections of the shorelinealong several kilometres of the popular sandy beach. The oil coated rubbish included leaves, wood and coconut shells.
Thai and foreign tourists quickly left the water to avoid being coated in the blackgunk.
At Hua Hin beach, tourists quickly left water to avoid black oil that floated ashore Tuesday coating everything.
Government workers and local residents began cleaning up Hua Hin beach on Wednesday after the Prachuap Khiri Khan coastline was fouled with oil, tar balls andpetroleum coated garbage. Blackened sand and oil coated objects were scooped up and disposed of in black bags hauled away by garbage trucks.
It was initially believed the oil had spilled from a large vessel far offshore but authorities are still investigating. On Tuesday, government workers used a patrol boat to search for the source of the spill. It is still unclear whether the oil came from adrilling platform or from a commercial vessel.
The areas hardest-hit by the oil spill were a stretch of about 10 kilometres from Khao Takiap to the Hua Hin fishing pier, and in front of the Klai Kangwon Palace. People have been advised not to go swimming at Hua Hin beach until advised that waterquality has returned to normal. However, the situation is improving. On Wednesday morning, oil slicks in the bay were not as thick as yesterday.