Almost 1,000 people were directly exposed to the gas and about 20-25 people are in critical but stable condition, said Kamal Kishore from the National Disaster Management Authority.

The gas has been identified as Styrene, a flammable liquid that is used to make a variety of industrial products, including polystyrene, fiberglass, rubber, and latex.

"When we arrived on the spot a lot of people were lying on the ground unconscious and we evacuated around 1,000 people and rushed them to the hospital," said Tej Bharath, a senior Vishakhapatnam district official.

Gopalapatnam Police helped hundreds of people to escape the apocalyptic scenes in ambulances, police vehicles, and state-provided buses, while others left on their own, said local police Inspector V Ramanayya.

At least 285 people are now in hospital, said K Kanna Babu, managing director of the state's disaster response force. Individuals were taken to hospitals across the city to be treated for exposure of the gas.

Babu said the district administration received the call by around 3:30 a.m. and his team was notified around 5:30 a.m. and were on-field by 6 a.m. But, he added, "we couldn't immediately enter because the smell of the gas was very pungent so we had to wait for half an hour before we could go in and start evacuating people."

The gas came out the factory's chimney and was carried by the wind, he said.

There are 10,000 people within the affected area of the gas leak; about 5,000 have been evacuated.

Disaster response teams have brought the leakage in the silo to a minimum and it is almost under control, authorities confirmed in a press briefing.

"Overall the situation is under control. Now, the situation is of rehab and treatment," said Pradhan.

How it happened

It is not immediately clear what led to the leak. However, the plant, which is owned by the South Korean company LG Chem, was preparing to reopen after coronavirus lockdown restrictions were eased, with the gas leak occurring during the process of re-starting operations, according to Bharath, the Visakhapatnam district official.

Reddy, the Andhra Pradesh minister, said workers at the plant had been conducting regular maintenance and gauging whether it was ready to return to full production. It was during this process that they found the leak coming from a storage tank, where the chemical had turned into a gas.

They immediately worked to neutralize the chemical, and had shut down the plant within an hour, Reddy said.

But Reddy said an alarm should have been raised when the gas leaked and asked why that didn't happen.

An LG Chem communications official told CNN that the plant's alarm only detects if raw Styrene is leaked in liquid form, and "something in there reacted," which meant it "leaked in vapor form."

Asked why it had turned into vapor, the official added: "That is something we need to investigate."

In a statement to CNN, LG Chem said it was taking measures to protect residents affected by the leak.

"(We) are currently assessing local town residents' damage situation and are taking maximum necessary measures for the protection of residents and employees together with related organizations," said the statement.

"The factory's gas leak is currently under control. Leaked gas can cause vomiting and dizziness from inhaling. (We) are seeking all measures so that related treatment can be done quickly."

"There is no specific antidote to reverse the effect of Styrene. The treatment does remain mainly supportive. Individuals have to be removed from the exposed area," said Dr. Randeep Guleria, Director at the All India Institute of Medical Sciences.

Local police are investigating the cause of the leak and conducting house to house visits in adjoining areas.

Photos of the aftermath has drawn parallels online with the Bhopal disaster -- a gas leak in the central Indian city of Bhopal in December 1984.

Nearly half a million people were exposed to toxic fumes, nearly 4,000 people died in the immediate aftermath, and around 10,000 subsequent deaths have been blamed on the leak, which is now considered one of the world's worst industrial disasters.

This leak is likely not as lethal as the Bhopal disaster, said Reddy, the state minister.

Government response

Indian Prime Minister Narendra Modi said in a tweet today that he had spoken with officials regarding Thursday's leak, and was monitoring the situation.

"I pray for everyone's safety and well-being in Visakhapatnam," he tweeted.

The state's chief minister is also set to visit the city hospital where residents are being treated, his office confirmed in a tweet.

"The Chief Minister is closely monitoring the situation and has directed the district officials to take every possible step to save lives and bring the situation under control," said the tweet.

The city's civic authority, the Greater Visakhapatnam Municipal Corporation (GVMC), warned residents to stay indoors during the response effort.

"There is gas leakage identified at LG Polymers in Gopalpatnam. Requesting Citizens around these locations not to come out of houses for the sake of safety precautions," GVMC tweeted. "As precautionary measures, the colonies and villages around the industry may leave to safer locations. Please use wet cloth as mask to cover nose and mouth."

Now as efforts turn from evacuation and rescue to investigation, state officials are beginning to look into the cause of the leak.

"Right now we are not taking any action but certainly the burden of proof lies with them (LG) -- to come forward and say what they have done," said Reddy. "We need to understand to what extent was this negligence or what it was. It will all come subsequently once we start ascertaining the situation on the ground."

He said that compensation of $131,000 per family will be given to those who have lost a loved one. LG will be asked to pay what it can and the state government will cover the rest, he added.

This story has been corrected to reflect that the plant has not yet reopened.