HONG KONG — The police on Friday detained personnel from a chemical plant in eastern China after an explosion killed at least 64 people, setting off an inferno that firefighters battled through the night and raising fears of toxic substances spreading in the air and water.

The blast Thursday afternoon at Jiangsu Tianjiayi Chemical Company, on the outskirts of the city of Yancheng in Jiangsu Province, was believed to have caused a tremor equivalent to a magnitude-2.2 earthquake, the China Earthquake Administration said.