State oil company Petroleos Mexicanos, or Pemex, said on Wednesday 58 workers had been hurt in the midafternoon blast in the industrial port city of Coatzacoalcos.
Veracruz state Gov. Javier Duarte told Radio Formula that three people had lost their lives. He also tweeted that 105 people in all were injured, including the 58 workers, the Associated Press reported.
The blast was felt as far as 6 miles (10 kilometers) away, Duarte said, adding that more than 2,000 people were evacuated from the area as a precaution.
By early evening the fire was reported under control, but Pemex still urged people to stay away from the area. Officials canceled Thursday's classes at area schools.
"The cloud that emanated from the PMV plant in Coatzacoalcos is dissipating rapidly, which means it is losing its toxic effects," the company said via its Twitter account.
The plant produces vinyl chloride, a hazardous industrial chemical that is used to make PVC pipes and for other purposes.
In early February, a fire killed a worker at the same facility.
Pemex said in a statement that the explosion happened at 3:15 pm at the Clorados 3 plant of Petroquimica Mexicana de Vinilo. It said the plant is operated by another company, Mexichem, in partnership with Pemex.
Pemex said 20 of the injured workers were treated in a company clinic in Coatzacoalcos and 38 were taken to nearby hospitals.
The Veracruz state Health Department said in a statement that two patients were in grave condition after suffering burns to their air passages from toxic gases. Others were treated for injuries including minor burns, contusions and broken bones.
There have been a number of accidents in recent years at facilities owned by Pemex or where it operates. Also in February, the company reported two people killed and eight injured in a fire on an offshore platform in the Gulf of Mexico.