Tens of thousands of people, many shouting "Death to America," have gathered in Tehran to mark the 1979 takeover of the U.S. embassy following the Islamic Revolution.
Observers say Monday's rally is the largest crowd in years at the annual event on the site of the former U.S. compound. Hardline conservatives had called for a major showing to protest President Hassan Rouhani's historic outreach to Washington.
A 15-minute telephone conversation after the U.N. general assembly between U.S. President Barack Obama and Mr. Rouhani in September was the first direct contact between the two countries' top leadership in over three decades.
Iran's supreme leader Ayatollah Ali Khamenei has voiced support for President Rouhani's overtures to the West, but says some of the president's moves were "not appropriate."
Islamist students stormed the U.S. embassy in Tehran 34 years ago, holding 52 hostages for 444 days, rupturing diplomatic relations and provoking decades of mutual hostility.
President Rouhani, who took office in August, has made several gestures toward the U.S. in the hope of easing Western sanctions imposed on Iran for pursuing a controversial nuclear program.
His gestures have drawn deep skepticism from Iranian ultra-conservatives who view the U.S. as an arch-enemy and oppose making concessions on Iran's nuclear activities. Western powers accuse Iran of trying to develop nuclear weapons under cover of a civilian energy program, a charge Tehran denies.