China sentenced 13 drug dealers and traffickers to death at a public rally, in the latest example of authorities using mass trials to support crime crackdowns.

The prisoners were among 38 drug convicts to receive sentences before a crowd of 10,000 on Friday in the city of Lufeng in the southern province of Guangdong, the official Xinhua News Agency said.

Eight more were sentenced to death with a two-year reprieve, while the others received long prison terms on the UN’s International Anti-Drug Day.

While China has held mass sentencings to mark the date before, the event came a day after President Xi Jinping pledged "no rest until a sweeping victory" was achieved against drugs. The country has seen a more than six-fold increase since 2008 in the number of people using synthetic drugs, the Chinese Ministry of Public Security said Tuesday.

On Thursday, the U.S. Department of State criticized China’s use of mass sentencing in the far western region of Xinjiang, where more than 40 percent of the population is Muslim. Authorities sentenced three people to death in stadium filled with 7,000 onlookers in March of last year.

Guangdong — a province of more than 100 million — is China’s largest producer of methamphetamine, Xinhua said. More than one-third of the drug consumed in China was made in villages administered by Lufeng, about 167 kilometers (104 miles) northeast of Hong Kong.

Authorities have arrested more than 660 suspects and seized more than 3,300 kilograms (7,275 pounds) of methamphetamine in Lufeng over the past two years, Xinhua said.