The area was sealed off and residents were evacuated after many reported breathing difficulties. Four are being treated in hospital.

The source of the leak is not clear so far. One official suggested that gas may have spread from manholes.

PTI news agency quotes Ludhiana Deputy Commissioner Surabhi Malik as saying: "We are going to collect samples from manholes. It is quite likely that some chemical reacted with methane in manholes."

Rajinder Pal Kaur Chhina, a local legislator, told Reuters news agency that the incident had happened near a milk shop. "People who came to buy milk in the morning, fell unconscious outside," she said.

A team from the National Disaster Response Force has been sent to the site, in the Giaspura area of Ludhiana. There are factories nearby.

Industrial gas leaks are not rare in India.

Three years ago a gas leak at a chemical plant killed least 12 people in the city of Visakhapatnam, in the state of Andhra Pradesh.

In 1984 a chemical leak in a pesticide plant at the central city of Bhopal killed thousands of people, in what is acknowledged to be the world's worst industrial disaster.