"The misbehavior of the U.S. government indicates that the U.S. does not abide by any principles protecting rights of those criticizing the system," Iranian Foreign Ministry spokesman Bahram Qasemi said of the U.S. arrest of a black Muslim American journalist who works for Iran's Press TV.
"It definitely rattled people," a former senior U.S. administration official told The Wall Street Journal. "People were shocked. It was mind-boggling how cavalier they were about hitting Iran."
"The Americans happily say that these sanctions are unprecedented in history. Yes, they're unprecedented. And the defeat that the Americans will face will be unprecedented, God willing," Supreme Leader Ayatollah Ali Khamenei said.
"The United States is a regime formed on the basis of capitalism, racial discrimination and hegemonism," Major General Mohammad Ali Jafari said.
Economy Minister Bruno Le Maire also stressed that tensions with the U.S. over Iran revealed the need for the EU to "affirm our independence."
"We will not obey such sanctions," Turkish President Recep Tayyip Erdogan said, adding that his government saw Washington's move as "violating the global balance."
"U.S. bullying is backfiring," Foreign Minister Javad Zarif warned earlier on Monday.
"We are in the economic war situation," Iranian President Hassan Rouhani warned as new U.S. sanctions took effect. "We are confronting a bullying enemy."
Iran can't afford to show weakness and has no reason to believe Trump will deal honestly with it even it backs down. Besides, Iran is far from the most urgent threat to the United States.
The World Court at the United Nations has ordered that the United States remove sanctions targeting humanitarian trade, food, medicine and civil aviation.
"Even if Trump himself is interested in negotiations, the people around him are committed to not letting that happen," said the president of the National Iranian American Council.
The European Union's foreign policy chief said the other nations would "protect the freedom of their economic operators."
The administration is "pleased with this decision because Iran diverts its economic resources away from its people to spread violence and instability across the globe," the U.S. ambassador to Germany said.
"Currently, Iran and its nuclear program have turned into [one of] the most significant concerns of the EU," Federica Mogherini, the EU representative for foreign affairs and security policy, said Friday.
Weak nations "are erased from history, while the strong, for good or for ill, survive," the prime minister said.
The aid package, agreed to on Thursday, will support the public and private sector in a bid to save a plan known as the Iran Nuclear Deal.
The oil company cut ties with Iran to avoid new U.S. sanctions, the Iranian oil minister said on state television Monday.
Iranian President Hassan Rouhani and Foreign Minister Javad Zarif initially argued that the deal would bring a windfall of foreign investment, but that promise did not materialize.
United States Ambassador Robert "Woody" Johnson called on British leaders to join the U.S. in "turning up the pressure" on Iran, or face "serious consequences." Washington recently reimposed sanctions on Iran despite opposition from allies.
Despite Khamenei promising that war is not an option, he also said that dialogue with Washington is not on the table.
President Donald Trump has said he wants to achieve "world peace," but his rejection of the Obama-era nuclear deal may only lead to more conflict with Iran.
"We deeply regret the re-imposition of sanctions by the U.S., due to the latter's withdrawal from the Joint Comprehensive Plan of Action," the foreign ministers of the EU, Germany, France and the U.K. said Monday.
The U.S., Israel and Iranian dissidents say that Iran is "hunting its enemies in Europe."