Sheikh Qassim, in his late 70s, is a spearhead of Bahrain's opposition movement demanding an elected government in the Shiite-majority kingdom, which has been ruled for more than two centuries by the Sunni Al-Khalifa dynasty.
Qassim was stripped of his citizenship in 2016 on charges that include "serving foreign interests", an allusion to Shiite-ruled Iran. He has been under de facto house arrest ever since.
Nedal al-Salman, acting president of the Bahrain Center for Human Rights, and a relative of Qassim who requested anonymity both confirmed Qassim was in critical condition.
Authorities allowed his family to transport him to a hospital of their choosing after they declined state-sanctioned medical care.
The cleric's ailing health has for weeks been a point of contention between authorities and the Qassim family, who had refused the government's offer to transfer him to hospital over fears he may be detained and deported should he leave his residence.
While his family has not made public the details of his condition, rights groups say he has high blood pressure, diabetes and heart disease.
Located between regional arch-rivals Iran and Saudi Arabia, Sunni-ruled Bahrain is home to a Shiite majority that has long complained of political marginalisation.
Protests demanding an elected government have rocked the archipelago since 2011, sparking a state crackdown on political dissent.
Authorities have jailed dozens of high-profile activists and disbanded both religious and secular opposition groups.
They have also stripped hundreds of their citizenship, making many stateless, according to Amnesty International.
The Bahrain government accuses Iran of backing the protests and attempting to overthrow the government. Tehran denies any involvement.
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