Tens of thousands of mainly Shi'ite protesters rallied in Bahrain on Friday against proposals for closer ties with other Gulf Arab countries, a plan pushed by Saudi Arabia to contain dissent in Bahrain and counter Iran's regional influence.
In Iran, thousands of protesters also rallied against the plan, state television showed, and an influential cleric denounced the idea as an "ill-fated plot".
Protesters in Bahrain chanted "Down, down Hamad!", referring to their country's ruler, King Hamad bin Isa Al Khalifa.
"We are against everything Al Khalifa (ruling family) are doing and we do not want a Bahrain-Saudi union," one protester said.
Arab heads of state met in Riyadh on Monday to discuss a call by Saudi King Abdullah to unite the six-member Gulf Cooperation Council (GCC), but failed to agree on further integration. Talks are due to resume later this year.
Speculation has been rife that Saudi Arabia's main goal is a union with Bahrain, where anti-government protests led by majority Shi'ites have gripped the island state since last year.
"This plot is an ill-fated plot that is taking place with the American and Zionist (Israeli) green light but they should know that the people of Bahrain and the region, Muslims around the world and in Iran will never tolerate it," Iranian cleric Kazem Sediqi said in a Friday sermon broadcast live on state radio.
Iranian state television aired footage of thousands of people holding rallies around the country and chanting slogans against the ruling royal families in Bahrain and Saudi Arabia to protest against the proposed Manama-Riyadh union.
"Instead of surrendering to its own people, it (the Bahraini government) is surrendering its identity, with total abjectness, to another country," Sediqi said.
Tension between Iran and US-allied Gulf Arab states has been high in recent months, with Arab leaders accusing Tehran of fomenting Shi'ite Muslim unrest in Bahrain - a charge that Shi'ite Iran and the protesters deny.
The dispute worsened when Tehran denounced efforts by the Gulf Arab states to forge closer political and military union, largely to counter Iran's growing regional power.
Bahrain, which hosts the US Fifth Fleet, has been in turmoil since pro-democracy protests in February 2011, inspired by successful uprisings in Egypt and Tunisia.
The uprising in Bahrain has raised Saudi fears of an impact upon Shi'ites in its oil-producing Eastern Province.
Tehran, for its part, summoned the Bahraini charge d'affaires on Thursday to complain about a statement from the small Gulf island state that accused Iran of violating its sovereignty.