About 80 per cent of the children in two villages near the plant had tested positive for lead poisoning, some of them with lead levels 10 times the level considered safe in China.
A few hundred children in a third village, Luobosi, were being tested for lead poisoning on Tuesday.
Authorities shut down the factory on Tuesday citing safety reasons, following protests in the latest sign of growing anger over the country's rampant industrial pollution.
Dai Zhengshe, the mayor of Baoji city, who arrived at the plant on Monday, gave his assurance that the plant will not be reopened until health standards are met, China's official Xinhua news agency said on Tuesday.
"We had to make sure the gas in the pipeline was exhausted before the plant was finally shut down," he said.
Security remained tight at the plant, with about 100 policemen guarding the compound and on standby in police buses outside, according to The Associated Press reports.
"If they relocate us to these nearby places, who can guarantee that our babies will be safe?There is lead in the air, the air is polluted, everything is polluted"
Deng Xiaoyan, parent of affected child
A man surnamed Ma from Madaokou, the other of the badly affected villages located about 500 metres from the factory, said residents believed at least two villagers were taken from their homes by police on Monday night.
He said the Baoji city government sent officials to his village on Tuesday to try to pacify residents.
"They wanted to persuade us not to cause trouble, but they didn't provide any solution to our problems," Ma told The Associated Press.
Xinhua said the villagers had been enraged by the smelting plant's defiance of the August 6 order to suspend operations.
The smelter, in northwestern Shaanxi province, opened in 2006 and residents living within 500 metres were supposed to have been relocated by this year.
Besides the 100,000-tonne plant, the Dongling Group, which is China's fourth-biggest zinc producer, also runs two other zinc lines in a separate area of Shaanxi, with a combined annual capacity of 150,000 tonnes.
A company source told Reuters the smelter had been shut for repairs since late July.
Lead poisoning can damage the nervous and reproductive systems and cause high blood pressure, anaemia and memory loss.
It is especially harmful to young children, pregnant women and foetuses, and the damage is usually irreversible, according to the World Health Organisation (WHO).
Following the outrage, authorities promised to relocate hundreds of families within two years with the building of new homes about 5km from the plant starting last week, but residents were not reassured.
|Parents of more than 600 children found excessive levels of lead in their blood [AFP]
Deng Xiaoyan, a farmer in Sunjianantou, one of the affected villages, said a recent test showed her three-year-old daughter had high levels of lead.
"If they relocate us to these nearby places, who can guarantee that our babies will be safe?" she said.
She said she thought those houses would still be too close.
"There is lead in the air, the air is polluted, everything is polluted," she said.
Dr Pascal Haefliger, a health and environment expert with the WHO in Geneva, said lead stays in the body for years after exposure and continues to affect brain development and the nervous system in growing children.
"Medical treatment exists, but will not be successful in removing all the lead from the body," he said.
China's pollution and lax product safety standards have long been a source of tension and unrest, particularly when residents of pollution hotspots - dubbed "cancer villages" because of high disease rates - feel they are being ignored.