Thai police Thursday arrested four men -- a Thai, two Chinese and a Taiwanese national -- for possession of more than 230 kilograms (500 lbs) of heroin in Bangkok.
Acting on a tip-off, drugs police arrested the 39-year-old Thai in a hotel in a Bangkok suburb. A subsequent search of his car uncovered the huge haul of heroin sealed in nearly 700 packets, police said.
The Chinese, aged 53 and 43, were arrested alongside a 43-year-old Taiwanese at the capital's main airport before they could board a flight to Hong Kong, police said in a statement.
"They were charged with drugs possession and are under police detention awaiting further legal action," it added.
Police did not put a street value on the massive seizure.
In June Thailand incinerated 125 kg of seized heroin in an annual show of its efforts to combat rampant drug trafficking through its borders.
The United Nations Office on Drugs and Crime in Bangkok has said the production of opium was at "alarming levels" in the Golden Triangle area -- where the remote edges of Thailand, Myanmar and Laos meet.
While production is a fraction of what it was in the 1970s and 1980s, Myanmar's cultivation of the drug has increased in recent years, to some 690 tonnes in 2012 -- over 10 percent of the global total.