The US will unleash its “toughest ever” sanctions against Iran on Monday following a wave of protests across the oil-rich country.
The Trump administration is reinstating all sanctions removed under the 2015 nuclear deal, targeting both Iran and states that trade with it.
It will hit oil exports, shipping and banks, all core parts of the economy.
Thousands of Iranians chanting “Death to America” rallied on Sunday, rejecting calls for talks.
Iran's military was also quoted as saying it would hold air drills 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, which led to four decades of mutual hostility.
Before travelling to a campaign rally for the US mid-term elections, President Donald Trump said Iran was already struggling under his administration's policies.
“The Iran sanctions are very strong. They are the strongest sanctions we've ever imposed. And we'll see what happens with Iran, but they're not doing very well, I can tell you.”
What started this?
Washington is re-imposing the sanctions after Mr Trump in May pulled out of a 2015 accord aimed at curbing Iran's nuclear ambitions.
Washington also says it wants to stop what it calls Tehran's “malign” activities including cyber attacks, ballistic missile tests, and support for terror groups and militias in the Middle East.
