Bahrain's top Shiite religious authority, Ayatollah Isa Qassim, has been stripped of his nationality, the country's Interior Ministry said on Monday.
Qassim, 79, is considered the spiritual leader of the country’s leading opposition grouping, Al-Wefaq, which the authorities closed down last week.
One of the most senior Shiite religious figures in the Gulf, Qassim is also the representative in Bahrain of Ayatollah Ali al-Sistani, an Iraqi cleric whom many Shiites across the region consider their supreme religious authority.
The ministry charged that, since acquiring Bahraini nationality, Qassim had "established organizations following a foreign political inspiration and had played a major role in creating an atmosphere of sectarian extremism."
It was not immediately clear if Qassim, who was born to a fishing family on the island of Duraz in northern Bahrain, had in fact previously held another nationality.
The United States said it was "alarmed" by the move.
"We remain deeply troubled by the government of Bahrain’s practice of withdrawing the nationality of its citizens arbitrarily, the overall precedent that this case could establish, and the risk that individuals may be rendered stateless," US State Department spokesman John Kirby said.
Bahrain authorities revoked the nationality of 208 people in 2015, according to Amnesty International, which slammed the tactic as arbitrary and a breach of the country's international human rights obligations.
Al-Wefaq was one of the main opposition groups involved in the pro-democracy protest movement that took place in the Sunni-ruled, Shiite majority Gulf kingdom in 2011.
The protests, which came after popular uprisings toppled dictators in Tunisia and Egypt, were crushed with the backing of neighbouring Saudi Arabia. More than 100 civilians were killed, according to opposition figures.
Bahrain authorities accused the protesters of being a Shiite sectarian movement backed by Iran, Saudi Arabia's main regional rival.
Recent weeks have seen renewed moves against the opposition, leading UN Secretary General Ban Ki-moon to express concern.
Prominent rights activist Nabeel Rajab was remanded in custody last week for questioning on charges of circulating false information that could harm the country.
Earlier this month, an appeals court increased the sentence of al-Wefaq's jailed leader, Sheikh Ali Salman, from four to nine years after convicting him of attempting to overthrow the regime.
Salman, seen as the moderate opposition's most prominent leader, had originally been sentenced for insulting the Interior Ministry and inciting sectarian hatred and law-breaking.
His lawyers had complained that the trial was unfair and human rights groups, along with the UN and the US, called for his release.
Ban's spokesman, Stephane Dujarric, last week said the UN chief was "dismayed ... that human rights defenders and activists in Bahrain have been intimidated and even stripped of their citizenship for peacefully carrying out activities to promote human rights, as well as for legitimately exercising their rights to freedom of expression and freedom of association."