Bahrain has revoked the Sunni-ruled kingdom's most prominent Shia cleric of his citizenship.
An interior ministry statement accused Sheikh Isa Qassim of using his position to "serve foreign interests" and promote "sectarianism and violence".
The cleric, who holds the religious rank of ayatollah, has backed protests led by the majority Shia community for greater civil and political rights.
The US has said it is "alarmed" at the move, and Iran has condemned it.
A US State Department spokesman said Washington was "unaware of any credible evidence" to support the removal of citizenship.
It comes days after Bahrain's government suspended the leading Shia opposition grouping, Wefaq National Islamic Society, closing its offices and ordering its assets to be frozen.
Wefaq's political leader, Shia cleric Sheikh Ali Salman, is in prison and recently had his jail term increased to nine years, after being convicted in 2015.
A US diplomatic cable published by Wikileaks described Sheikh Isa Qassim as Wefaq's spiritual leader. He is also regarded as the spiritual leader of Bahrain's wider Shia community.