Many of those affected are complaining of a burning sensation in the eyes and difficulties breathing.
Thirteen people have died, with hundreds of others taken ill, after a gas leak in south India.
The leak, in the city of Visakhapatnam in Andhra Pradesh state, has been traced to the LG Polymers plant.
Doctors say "hundreds" of people have been taken to hospital - many complaining of a burning sensation in the eyes and difficulties breathing.
The incident, which took place around 03:00 local time (21:30 GMT), may have been due to negligence, officials say.
The leak occurred when the plant was being re-opened for the first time since 24 March when India went into lockdown to curb the spread of coronavirus.
The state Industries Minister Goutam Reddy told BBC Telugu that it looked as though proper procedures and guidelines were not followed when the plant was being re-opened.
Swaroop Rani, a senior police official in Visakhapatnam, told AFP that the plant had been left idle because of the lockdown.
"[The gas] was left there because of the lockdown. It led to a chemical reaction and heat was produced inside the tanks, and the gas leaked because of that."
Local villagers complained around 03:30 and police immediately went to the scene, but had to quickly retreat for fear of being poisoned, the news agency quoted her as saying.
"One could feel the gas in the air and it was not possible for any of us to stay there for more than a few minutes," she said.
As the gas spread, residents ran out of their homes in panic. Distressing images of people fainting and dropping unconscious on the streets are being shared on social media.
Some factory employees are believed to have been inside when the leak occurred, but officials say they have no information about them.
It is feared that the fumes have spread over a radius of about 3km (2 miles) and officials have been evacuating people from surrounding areas.
A district health official said at least 1,000 people had been sent to hospitals and many others were feared to be unconscious in their homes.
"There are people who were sleeping," AFP quoted BK Naik as saying. "We are working to get people to the hospital. They need oxygenation and fresh air," he said.
A senior district official said that initial attempts to control the gas leak were unsuccessful. However, local news agencies have reported that the situation is now under control.
Meanwhile, Rajendra Reddy, a senior official in the Andhra Pradesh Pollution Control Board, told the BBC that the leaked gas was styrene, which is usually refrigerated.
"We are trying to understand the long-term impact of the chemical on those who have inhaled it during the leak," he said.
In the meantime, officials have asked people to protect themselves by covering their faces with a wet cloth.
India has a tragic history of gas leaks.
In 1984, a toxic chemical leak in a pesticide plant in the central city of Bhopal killed thousands of people, in what is acknowledged to be the world's worst industrial disaster.
More than 35 years later, victims say children are still being born with disabilities because of the effects of the methyl isocyanate gas spill.
Reporting by BBC Telugu's Satish Balla and Deepthi Bathini
What is Styrene and how can exposure affect humans?
- Styrene gas is a colourless, or light yellow, flammable liquid primarily used in the production of polystyrene plastics and resins - it is used in the manufacture of containers for foodstuffs, packaging, synthetic marble, flooring, disposable tableware and moulded furniture
- Breathing air contaminated with styrene vapours can cause irritation of the nose and throat, coughing and wheezing, and create a build-up of fluid in the lungs
- Exposure to larger amounts can result in the onset of "styrene sickness", the signs and symptoms of which include headache, nausea, vomiting, weakness, tiredness, dizziness, confusion and clumsy or unsteady motion (known collectively as central nervous system depression)
- In some cases exposure to styrene can also result in irregular heartbeats and even coma
- Several epidemiologic studies suggest there may be an association between styrene exposure and an increased risk of leukemia and lymphoma though the evidence is inconclusive
Information from the PHE Centre for Radiation, Chemical and Environmental Hazards and US Environment Protection Agency