Bahrain's most senior Shia cleric Sheikh Isa Qassim has been taken to a private hospital in critical condition after weeks of suffering from deteriorating health.

Sheikh Qassim's family and the Bahrain Center for Human Rights group confirmed his hospitalization on Monday and said he was in "critical condition."

He has been under house arrest for around 500 days amid daily protest gatherings in the northwestern village of Diraz which is besieged by regime forces.

Bahrain's leading political party al-Wefaq and rights bodies said the cleric, who is in his late 70s, was suffering from high blood pressure, diabetes, and heart disease.

Deputy Secretary-General of al-Wefaq Sheikh Hussein al-Daihi said the spiritual leader was taken from his home to a private hospital amid tight security measures.

The cleric, known as the spiritual leader of the country's Shia Muslim majority, has been hailed as a source of inspiration for their peaceful demonstrations against the ruling Al Khalifa regime.

The monarchy has killed scores of people and imprisoned hundreds of people among the protesters, who are campaigning for an elected government and a stop to discrimination against the Shias.

Back in June, Sheikh Qassim faced potential expulsion from Bahrain after authorities revoked his citizenship, accusing him of “illegal fund collections, money laundering and helping terrorism.” He has strongly rejected all the allegations.

United Nations spokesman Stephane Dujarric demanded last Thursday that Sheikh Qassim be released, and asked for immediate medical attention for the cleric.