The crowds also sent a message to Iran's Supreme Leader Ayatollah Ali Khamenei, who has cautiously 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 that could put pressure on Khamenei to reconsider his backing of Rouhani's groundbreaking exchanges with the U.S.

In September, Rouhani accepted a call from President Barack Obama following the United Nations General Assembly in New York, where Secretary of State John Kerry held talks with Iran's foreign minister. Ties between the two countries were severed in 1979 when, ten months after the fall of the U.S.-allied shah, students stormed the U.S. embassy, taking hostage 52 staff for 444 days. There have been no U.S.-Iranian diplomatic relations since.

Critics of U.S.-Iran dialogue made their views immediately, 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. Others carried well-known banners reading "We trample America under our feet" and "The U.S. is the Great Satan." One image showed Obama in a wrestling uniform with Star of David earrings, symbolizing Israel.

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," said Khamenei, according to the official IRNA news agency.

Iran seeks to have painful economic sanctions eased in exchange for concessions in its nuclear program. The West fears Iran's uranium-enrichment program could eventually produce weapons-grade material. Iran insists it only seeks reactors for energy and medical applications, but has not allowed international inspectors to visit its sites.

Outside the former embassy's brick walls – covered with anti-U.S. murals – students carried a model of a centrifuge used in uranium enrichment. A slogan on it read: "Result of resistance against sanctions: 18,000 active centrifuges in Iran."

Another banner quoted Khamenei: "The aim of sanctions is to make the Iranian nation desperate."

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 the Iranian 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.

Al Jazeera and wires