Iranian state-run media say thousands of Iranians took part in protests denouncing plans for a union between Saudi Arabia and Bahrain.
Authorities had called on citizens to take part in the protests on May 18 against what the Iranian government has called an "American plan to annex Bahrain to Saudi Arabia."
State-run television footage showed demonstrators in Tehran shouting anti-American slogans.
An influential cleric, Ayatollah Kazem Sediqi, in a sermon broadcast live, denounced the planned union as an "ill-fated plot."
Tensions between Iran and U.S.-allied Gulf Arab states have been running high, with Arab leaders accusing Shi'ite-led Iran of fomenting unrest in majority Shi'ite Bahrain, which is ruled by a minority Sunni monarchy.
Saudi Arabia last year sent troops to Bahrain to help crush a Shi'ite uprising.
Based on reporting by Reuters and AFP