Hundreds of furious Chinese parents rampaged through a lead factory yesterday after the authorities blamed pollution from the plant for poisoning more than 600 children.

The protest follows a week of denials and revelations that have grabbed headlines in a country increasingly fearful for its environmental health.

According to the Xinhua news agency, villagers tore down a 300-metre fence around the Dongling Lead and Zinc Smelting Company in Fengxiang county, Shaanxi province, and smashed up coal trucks after the government ruled that emissions from the facility had harmed the health of local people.

At least 615 of the 731 children in two nearby villages have been diagnosed with dangerously high amounts of lead in their blood. A gradual build up of lead in the bloodstream can lead to anaemia, muscle weakness and brain damage. More than 140 children have been treated in hospital.

The authorities are being blamed in part for the incident. The risks were known when the factory started operations in 2003 but the local government was desperate for investment. It promised to rehouse nearby villagers, but after 100 were moved out in 2004 the relocation plan stalled, apparently because of insufficient funds.

Locals appear to have been only vaguely aware of the dangers. After a recent study showed how prevalent the poisoning had become, hundreds of families took their children to hospitals for tests.

Xue Yani told domestic reporters she was worried about her eight-year-old son because he was so short and thin he looked liked a four-year-old. "I just want him to have a check-up so that we can find out what's wrong." The doctors revealed that the boy's blood contained 239 micrograms of lead per litre, almost five times the safe level.

Workers are more severely affected. Ma Yungang, a 23-year-old employee of the smelting plant, required a transfusion after a litre of his blood was found to contain 1,100 micrograms of lead. The factory was ordered to suspend operations at the start of this month, but the local government has been reluctant to close a business that accounts for more than a sixth of the local economy.

The firm has never been accused of breaking emissions regulations and last week, the local environmental bureau said waste discharges met national standards along with the amount of pollution in the nearby water and soil.

Ma Tianjie, toxics campaigner for Greenpeace in China, said local environment bureaus often lacked the funds to make expensive tests of heavy metals, which are not obligatory under national regulations. "This case is not unique in China. There is heavy metal pollution in a lot of places." Yesterday, however, the authorities reversed their position and confirmed Dongling as the cause of the lead poisoning cases.

The government has promised to pay for the treatment costs of the victims, but parents are unlikely to be easily mollified. Relocation plans have been accelerated but this has also run into complications because the site of the new homes has also been found to be polluted. Local authorities declined to comment.