Kraft Foods factory in Bad Fallingbostel (Screengrab/Reuters)

A chemical leak at a Kraft Foods factory in Germany has prompted a mass evacuation of 1,800 people.

A cloud of poisonous gas rose over the plant in the small town of Bad Fallingbostel, near Hanover in Lower Saxony, northern Germany, but has started to dissipate.

More than 700 firefighters tried to cool a chemical tank after factory workers poured 10,000 litres of nitric acid into it. The container was holding 14,000 litres of sodium hydroxide solution and the accidental mix provoked a chemical reaction.

Experts feared that a jump in temperature could cause an explosion. The nitrous gases released by the chemicals are extremely dangerous for the respiratory system and the skin.

Homes, schools and nurseries have been closed and local authorities have set up an exclusion zone of 500 metres.

"There has at no time been any danger for the population because we decided, along with the authorities, to evacuate as a precautionary measure," Heike Hauerken, spokeswoman for the company, told news channel N24.