The US will unleash its "toughest" sanctions on Iran Monday after a wave of protests in the oil-rich country.
The Trump government is recovering all sanctions imposed under the 2015 Nuclear Agreement and targeting both Iran and states acting with it.
It will hit oil exports, shipping and banks, all core sectors of the economy.
Thousands of Iranians who sang "Death to America" gathered on Sunday and rejected calls for talks.
The Iranian military has also been quoted as conducting aerial exercises on Monday and Tuesday to prove the country's defensive capabilities.
The demonstrations took place on the 39th anniversary of the occupation of the US Embassy in Tehran, resulting in four decades of hostilities.
President Donald Trump said Iran is already fighting under its government's policies before traveling to an election campaign on US presidential elections.
"Iran's sanctions are very strong, they are the strongest sanctions we've ever imposed, and we'll see what happens to Iran, but they're not doing very well, I can tell you."
What started this?
Washington re-imposes sanctions after Mr Trump made an agreement in 2015 in May to stem Iran's nuclear ambitions.
Washington also says it intends to halt Tehran's "malicious" activities, including cyber attacks, ballistic missile testing, and support for terrorist groups and militias in the Middle East.
"We are working diligently to ensure that we support the Iranian people and direct our activities toward changing the malicious behavior of the Islamic Republic of Iran," US Secretary of State Mike Pompeo told Fox News on Sunday.
"That's the goal, that's the mission, and we'll do that on behalf of the president."
What could be the influence?
The US has gradually imposed sanctions, but analysts say this last round is by far the most significant.
The sanction list will include more than 700 individuals, businesses, ships and aircraft, including major banks, oil exporters and shipping companies.
Mr Pompeo said more than 100 major international companies have withdrawn from Iran over imminent sanctions.
He also said that Iran's oil exports fell by nearly a million barrels a day and stifled the country's main source of finance.
In addition, the Brussels-based Swift International Payments Network is expected to cut ties with targeted Iranian institutions and isolate Iran from the international financial system.
How did the EU states react?
The United Kingdom, Germany and France, which are among the five countries still committed to the Nuclear Pact, have all objected to the sanctions.
They have promised to support European companies that have "legitimate business dealings" with Iran and set up an alternative special purpose vehicle (SPV) that will help businesses trade without US fines being imposed.
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However, analysts doubt that this will significantly reduce the impact of sanctions on Iran.
US Treasury Secretary Steven Mnuchin said in recent days that the US would "aggressively" target any company or organization that "bypassed our sanctions."
Who is freed?
The Trump Administration has granted exemptions to eight countries to continue importing without mentioning Iranian oil.
Its US allies Italy, India, Japan and South Korea include Turkey, China and India.
Mr Pompeo said that countries had already made "significant reductions in crude oil exports" but needed "some more time to get to zero".
He said that at some point two would stop imports and the other six would reduce them significantly.
What was the reaction in Iran?
US sanctions coincide with the US Embassy's siege on November 4, 1979, which took place shortly after the overthrow of the US-backed Shah.
Some 52 Americans have been held hostage in the Embassy for 444 days, and the two countries have been enemies ever since.
Hardliners hold protests every year to siege the siege, but on Sunday, demonstrators have also raised their anger over the sanctions.
Millions of people in cities and communities have been sent out in the Iranian state media, swearing allegiance to Supreme Leader Ayatollah Ali Khamenei, although the BBC was unable to independently verify this number.
It was followed by a fiery speech by Ayatollah Khamenei on Saturday warning the US not to reestablish "domination" over Iran prior to 1979.
However, some Iranians have gone to Twitter to express their frustration with the regime. The hashtag #Sorry_US_Embassy_Siege attracted over 19,000 tweets.
A user has tweeted in English: "For the past 40 years, Iran's Islamic regime has been trying to portray the US and Israel as enemies of Iran, but the Iranian people do not think like mullahs, we love all nations and all the people of the world. "
Another said, "America is not our enemy, our enemies have taken us hostage at home [country], "