Indian firebrand nationalist politician Bal Thackeray, who founded the right-wing Hindu party Shiv Sena, died on Saturday after suffering a cardiac arrest, his doctor said.

Thackeray, 86, has "breathed his last", his physician Jalil Parkar told a huge crowd assembled outside the veteran politician's house in India's financial hub of Mumbai.

Thackeray, regarded as one of India's most divisive politicians, had been placed on a ventilator earlier after his health deteriorated sharply, and Parkar said attempts to revive him after he suffered a cardiac arrest failed.

Mumbai police have already stepped up security to prevent unrest in the event of the death of the politician, whose Shiv Sena party became synonymous with deadly communal violence during its campaign to protect local Marathi-speaking people and their culture from migrant workers.