Four people were arrested and more than 8 million methamphetamine pills they were delivering to Bangkok seized in Lop Buri province on Tuesday.
Three men, Witthaya Moonsarn, 28, Natthaphol Promwong, 23 and Pheeraphol Deesorn, and a woman, Panita Piromsuparp 25, were caught at a PTT petrol station on Saraburi-Lom Sak Road in Phatthana Nikhom district of Lop Buri.
Metropolitan Police Bureau commissioner, Pol Lt Gen Pakapong Pongpetra, said they were travelling in two cars. A search found 21 sacks containing a total of 8.41 million speed pills in the vehicles.
He said the arresting officers were acting on information received following arrests in several other major drug cases in Bangkok.
The information led them to Mr Witthaya. He allegedly headed a gang moving illicit drugs from the Northeast to customers in the Central Plains, using a Honda Jazz car with Bangkok licence plates and a Toyota Revo pickup truck with Prachuap Khiri Khan licence plates.
Pol Lt Gen Pakapong said the gang often used Highway 21 as a smuggling route.
Investigators had closely monitored their activities. Mr Witthaya and his accomplices had travelled to Nong Khai on Monday.
Officers believed they went there to pick up the drugs.
During questioning, the suspects allegedly admitted they were paid 100,000-200,000 baht each time they made a drug delivery.
They were charged with illegal possession of drugs with intent to sell and handed over to narcotics suppression police for legal action.