Well-known social activist Medha Patkar, a Hindu, said Thackeray's latest anti-Muslim outburst was the most extreme ever by any Hindu fundamentalist leader.
"Thackeray should be taken to task, otherwise he will become uncontrollable. This just shows up the fascist that he is," Patkar told AFP.
Thackeray heads the Hindu militant Shiv Sena party -- the third largest partner in Prime Minister Atal Behari Vajpayee's federal coalition.
In an interview with his party newspaper, Saamna, on Sunday, Thackeray said India's 120 million Muslims -- the country's largest religious minority -- should not be allowed to vote.
"If you have the guts, disenfranchise Muslims. Let's see who will cry for them. Everyone will then become pro-Hindu," Thackeray said.
Muslims in India, which is predominantly Hindu, will continue to be "uppity" as long as political parties woo them for votes," he added.
"Muslims are taking the most advantage of this term 'secularism.' They are converting Hindus to Islam, propagating their religion," he said.
In response, Patkar said "the Hindu religion and Hindus in general are not as intolerant as he is. He should be publicly condemned and mass protests should be organised against him."
Anti-fundamentalism activist Asgar Ali Engineer, a Muslim, said Thackeray's statement should serve as a warning of the dangers of unfettered Hindu nationalism.
"Thackeray is indicating what the hardline Hindu parties will do if they come to power on their own. They will make India into a Hindu theocratic state with all other religious minorities reduced to second-class citizens," Engineer said.
Inflammatory interviews and editorials are something of a Thackeray trademark. In July, he was arrested in Bombay on charges of inciting communal riots that left 1,000 people dead.
The charges related to editorials Thackeray wrote in Saamna during 1992-93 Hindu-Muslim riots in Bombay. He was released just hours after his arrest after a court ruled that the charges had passed their three-year statute of limitations.