Jayaram Jayalalithaa, the chief minister of the southern state of Tamil Nadu and one of India’s most powerful politicians, was in “very critical” condition Monday, a day after she suffered a cardiac arrest, according to authorities at the hospital where she is being treated.

Ms. Jayalalithaa, who has a cult-like following, was on life support systems, Apollo Hospitals said in statement Monday afternoon. Four doctors from New Delhi’s All India Institute of Medical Sciences were rushed to the state capital, Chennai, India’s Health Ministry said.

Dozens of supporters gathered outside the hospital premises, some weeping, others praying. C.R. Saraswathi, a spokeswoman for the chief minister’s party, said the news of the heart attack had come as a “major shock” to her followers. After months of medical treatment, Ms. Jayalalithaa was recovering, party workers had been told last week.

In Indian politics, few leaders match Ms. Jayalalithaa’s devoted and fanatical support base. After she was convicted in 2014 on charges that she had amassed disproportionate wealth, newspapers reported suicides by devastated followers. When the court’s decision was overturned the next year, supporters celebrated by pouring milk on giant posters–a Hindu ritual usually reserved for deities. They held hundreds of religious prayers and danced in the streets.

On Monday, India’s Home Ministry said it was prepared to provide extra security personnel if news of her ill health provoked anger and violence. Parliamentarians from Ms. Jayalalithaa’s party in New Delhi and state lawmakers from across Tamil Nadu made their way to Chennai.

Ms. Jayalalithaa, fondly called “Amma” or mother in Tamil, is ubiquitous in the state. Posters bearing her picture are plastered across cities and in the countryside. Through “Amma canteens,” her government serves hundreds of thousands of highly subsidized meals each day. “Amma pharmacies” offer discounts on medicines.

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Well wishers of Ms. Jayaraman gathered outside a hospital where she is being treated in Chennai, India, Dec. 5, 2016. PHOTO: REUTERS

The daughter of an actress in Tamil-language films, Ms. Jayalalithaa’s first career was in showbiz. She featured in more than 100 movies that catapulted her to fame before she turned to politics in the 1980s.

Her party, the All India Anna Dravida Munnetra Kazhagam, is now a major force in India’s Parliament. Following elections in 2014, it emerged as the second-largest opposition party, narrowly trailing the incumbent Congress party.

Ms. Jayalalithaa’s political life hasn’t been free of scandal. She had to step down from her position as chief minister after her 2014 conviction. In that judgment, the court said she had accumulating wealth well beyond her known sources of income while leading Tamil Nadu. Last year, a higher court acquitted Ms. Jayalalithaa, allowing her to return to office.

Major Indian politicians, including several state chief ministers, responded to the news of her ill health on Twitter. Indian President Pranab Mukherjee said he was “distressed,” while Congress Vice President Rahul Gandhi tweeted that he was praying for her quick recovery.