At least six people were killed and several others were seriously injured in an explosion Thursday at a Czech chemical plant in the northern town of Kralupy-nad-Vltavou, firefighters said. "We have received information about six dead and several others seriously injured," regional firefighter spokeswoman Vladimira Kerekova said, quoted by the CTK news agency.  The Prague-Vinohrady hospital has triggered its emergency plan and is getting ready to receive a "significant number" of patients, she added. Petr Holecek, the towns mayor, told Czech public television a tank with an unspecified substance exploded. Speaking from the site of the explosion, Holecek also said he believed there was no danger to the town. Emergency services spokeswoman Petra Effenbergerova said six people were injured. Kerekova said the factory belongs to the Polish firm Synthos, which manufactures synthetic rubber and polystyrene. "We have launched an investigation to determine the circumstances and causes," police spokeswoman Marketa Johnova said. She said police had learnt of the explosion at 10 am local time. No dangerous substances have leaked out, according to Jonova. Two people were injured in an explosion at the same factory in 2015.