It was not immediately clear what caused the incident, which took place mid-afternoon in Suphan Buri province about 120 km (74.56 miles) north of Bangkok, police said.
"We cannot tell whether all the workers were killed. The officials are entering the site to investigate. They have not found any survivors," Provincial governor, Nattapat Suwanprateep, told Reuters.
A preliminary investigation put the death toll at 20, according to a police statement, with many injured.
Prime Minister Srettha Thavisin was briefed on the explosion while at the World Economic Forum in Davos, Switzerland and urged a swift investigation.
"The prime minister ordered an inspection of the plant to see if it was operating legally and if the explosion was caused by negligence. The law must be enforced to the fullest extent because there are innocent deaths and injuries," Srettha's office said.
