DUBAI (Reuters) - Iran's supreme leader has blasted as "foolishness" a decision by Bahrain's leaders to strip a top Shi'ite Muslim cleric of his citizenship, and said it could provoke violence from Shi'ites, who make up the majority in the Sunni-ruled Gulf kingdom.

The speech by Ayatollah Ali Khamenei, carried by state media, came after Bahrain's Sunni authorities stepped up measures against the island's Shi'ites and stripped their spiritual leader, Ayatollah Isa Qassim, of his citizenship.

"This is blatant foolishness and insanity. When he still could address the Bahraini people, Sheikh Isa Qassim... would advise against radical and armed actions," Khamenei said in remarks carried by state television on Sunday.

"Attacking Sheikh Isa Qassim means removing all obstacles blocking heroic Bahraini youths from attacking the regime."

Bahrain in 2011 put down an uprising led by Shi'ites demanding reforms. Since then, there have been almost daily skirmishes between Shi'ite youths and security forces, as well as several bomb attacks.

Bahrain accuses Iran of fomenting unrest among its Shi'ite population and providing financial and material support, a charge Tehran denies.

In a series of moves over the past three weeks, authorities also closed down the main Shi'ite opposition al-Wefaq Islamic Society, doubled the prison sentence on the group's leader, Sheikh Ali Salman, and detained prominent rights campaigner Nabeel Rajab.

(Story refiles to remove extraneous word 'island' in second paragraph.)

(Reporting by Dubai newsroom; Editing by Raissa Kasolowsky)