Iranian President Hassan Rouhani has said that Iran will not yield to the "language of threats" and will "strongly confront any war-mongering policies" of the new US administration.
Mr Rouhani made the comments in a public speech in Tehran on Friday as hundreds of thousands of people took part in rallies marking the the anniversary of the 1979 Islamic Revolution.
State TV showed people carrying pictures and effigies of US President Donald Trump, burning US and Israeli flags and shouting 'Death to America!" as Supreme Leader Ayatollah Ali Khamenei called on citizens to show Iran is not afraid of American "threats".
Some carried pictures of Mr Trump, as well as British Prime Minister Theresa May and Israeli Prime Minister Benjamin Netanyahu captioned "Death to the Devil Triangle".
"Some inexperienced figures in the region and America are threatening Iran... They should know that the language of threats has never worked with Iran," President Rouhani told a crowd in Azadi Square.
"They should learn to respect Iran and Iranians ... We will strongly confront any war-mongering policies."
Other Iranians marched more peacefully. On Facebook and Twitter, although they are banned inside the country, the hashtag '#LoveBeyondFlags' trended as users urged for an end to the tradition of US flag burning. Many more thanked Americans who protested against Mr Trump's so-called 'Muslim ban.'
Relations between Tehran and Washington have soured quickly since Mr Trump took office just a few weeks ago.
Mr Trump has put Iran "on notice" in reaction to a January 29 Iranian missile test, and imposed fresh sanctions on several individuals and entities on terrorism-related issues.
The Iranian Foreign Ministry has maintained that the test did not violate the terms of a UN resolution on nuclear warheads, saying that the country will not allow its domestic security to become the “focus of international debate”.
The president's proposed ‘Muslim ban’ - which is currently being challenged in federal courts - targeting citizens of Iran and six other Muslim countries was met with outrage in Tehran, which quickly imposed a retaliatory travel ban for US citizens and announced Iran’s central bank would stop using the US dollar for official and financial reporting.
Earlier this week Ayatollah Khameni said Mr Trump had made it easier to see the "real face of America."
“What we have been saying, for over thirty years, about political, economic, moral, and social corruption within the US ruling establishment, he came out and exposed during the election campaigns and after the elections.
"Now, with everything he is doing - handcuffing a child as young as five at an airport - he is showing the reality of American human rights."
On the campaign trail the then-candidate heavily criticised the stance of his predecessor Barack Obama towards Iran. He said he would consider scrapping the historic 2015 nuclear deal, which eased decades of crippling sanctions in exchange for curbs to Iran's nuclear programme.
While the administration has dampened down talk of scrapping the deal, US National Security Adviser Michael Flynn said last week that the international community had been “too tolerant of Iran's bad behaviour,” including several other missile tests in the last 18 months.
“The days of turning a blind eye to Iran's hostile and belligerent actions towards the United States and the world community are over,” he said.
“The amateur and irrational policies of the new US administration will change nothing about the principles of Iranian politics,” it read.
Reuters contributed to this report