Such protests occur every year outside the former embassy compound to mark the anniversary of the 1979 takeover following the Islamic Revolution. But the latest demonstration is the largest in years after calls by groups such as the powerful Revolutionary Guard for a major showing, including chants of "death to America" that some of Rouhani's backers have urged halted.
The crowds also send a message to Iran's Supreme Leader Ayatollah Ali Khamenei, who cautiously has backed Rouhani's overtures to the U.S. and efforts to end the impasse with the West over Tehran's nuclear program.
Opponents of thawing relations with the U.S. say they will not back down, opening the prospect of deeper internal rifts and tensions that could put pressure on Khamenei to reconsider his backing of Rouhani's groundbreaking exchanges with the U.S.
In September, Rouhani accepted from a call from U.S. President Barack Obama following the annual U.N. General Assembly in New York, where U.S. Secretary of State John Kerry held talks with Iran's foreign minister. Ties between the two countries were severed after the embassy siege, which began a hostage crisis with 52 people held for 444 days.
Critics of the dialogue made their views immediately known, hurling insults and eggs at Rouhani's entourage upon their return from New York. Late last month, huge banners appeared around Tehran depicting the U.S. as a sinister and deceitful adversary that seeks to weaken Iran. Tehran officials ordered the signs removed, but they appeared in poster form at the demonstration Monday outside the former embassy compound.
Protesters also stomped on images of Obama and the U.S. flag.
On Sunday, Khamenei appeared to chide hard-liners by denouncing any attempts to undermine Iran's nuclear negotiators. Talks with world powers are scheduled to resume Thursday in Geneva.
Diplomats "are on a difficult mission and nobody should weaken those who are on assignment," the official IRNA news agency quoted Khamenei, who has final say on all matters of state, as telling a group of students.
Iran seeks to have painful economic sanctions eased in exchange for concessions in its nuclear program to address concerns by the West, which fears Iran's uranium enrichment could eventually produce weapons-grade material. Iran insists it only seeks reactors for energy and medical applications, but has not made public its possible confidence-building offers at the talks.
Khamenei's backing of Rouhani also puts him in an unfamiliar spot of having to reassure hard-liners he has not abandoned their views.
Khamenei on Sunday praised Iranian militant students who stormed the U.S. Embassy in 1979.
"Thirty years ago, our young people called the U.S. Embassy a 'den of spies.' ... It means our young people were 30 years ahead of their time," he said, a reference to a series of reports of U.S. eavesdropping on foreign leaders, including German Chancellor Angela Merkel.