The deaths were reported at Chhatrapati Shivaji Maharaj Memorial Hospital in Kalwa, Thane, about 25km north-east of Mumbai – the country’s financial capital.
The hospital is the only major tertiary medical centre in the area.
Over 24 hours from Friday evening, 18 patients at the hospital died, including 13 in the intensive care unit.
Maharashtra Chief Minister Eknath Shinde said the situation was unfortunate and has set up an independent investigation committee.
“The government has taken the incident very seriously and a committee will be appointed under the chairmanship of the Director of Health to conduct a thorough investigation. Action will be taken according to the report,” Mr Shinde said.
Twelve of the deceased were over the age of 50. Officials said 10 women and eight men were among those who died at the hospital.
The families of the patients have made allegations of negligence and said there were shortages of oxygen.
They have demanded the suspension of doctors and medical staff and are seeking compensation from the government.
Abhijit Bangar, Municipal Commissioner in Thane, said several patients were referred to the hospital in critical condition.
“The patients had complications of kidney stones, chronic paralysis, ulcer pneumonia and kerosene poisoning, among other conditions,” he said.
“The line of treatment will be probed and statements of kin of the deceased will be recorded.
"Some patients were referred to the hospital from other hospitals in a critical condition.”
Five patients died at the same hospital on August 10.