Large crowds are gathering outside a Chennai hospital after one of India's most popular and powerful politicians, J Jayalalitha, suffered a heart attack on Sunday night.

The news has sparked fresh fears about her condition as Ms Jayalalitha has been receiving treatment for months.

Last seen in public in September, news of her health has been closely guarded.

The former film star has been Tamil Nadu chief minister three times and is virtually revered in the state.

With the extreme devotion she inspires among her supporters, many of whom refer to her as "Amma"(mother), there are concerns that a serious deterioration in her condition could spark unrest.

Police have been put on alert across Tamil Nadu and outside her official residence.

News of her health over the last few months has been closely monitored and guarded, and in a sign of the sensitivity around the issue two people were arrested for spreading rumours about her health in October.

A statement from the Apollo Hospitals said that she "suffered a cardiac arrest" on Sunday evening and that she was being monitored by a team of experts.

The hospital, which is making regular updates on Twitter, said in its latest statement that "doctors are closely monitoring Hon'ble CM's condition and they are trying their very best".

Earlier on Sunday evening, the hospital had asked the public to "pray for her speedy recovery".

Leaders, including President Pranab Mukherjee have tweeted, expressing concern about her health.

Last month a letter released by her party seemed to hint at a recovery for the politician saying she had been "reborn" thanks to prayers from her supporters.

As it also urged people to vote for her party at upcoming by-elections, critics said it could have also been an attempt to quell fears about the extent of her illness and mobilise her supporters.