The new American sanctions bar any transactions with Iran involving dollars, gold, precious metals, aluminum, steel, commercial passenger aircraft and coal. They also end imports into the United States of Iranian carpets and pistachio nuts, both of which are important symbols for Iranians. Another round of sanctions is due to be introduced on Nov. 4.

The other countries that signed the 2015 nuclear agreement — Britain, China, France, Germany and Russia — do not support the United States sanctions, which Secretary of State Mike Pompeo said in May would be the “strongest in history.”

The Iranian government tried to calm the markets on Monday before the sanctions were scheduled to begin, although it was unclear how effective those measures would be. Iran’s currency, the rial, rose modestly for the day, but it has lost more than 80 percent of its value in the past 12 months. And while much of the devaluation stems from mismanagement and corruption, the United States’ withdrawal from the nuclear agreement has exacerbated the rial’s weakening.

Iran’s new central banker, Abdolnaser Hemmati, on Sunday evening spoke of “economic warfare” by the United States, but also said in an interview on state television that Iran was “powerful.”

In an attempt to ease the economic hardship in the country, he announced that starting on Monday, Iranians would no longer be restricted in the amount of dollars and gold they could bring in. An earlier rule capped that amount at 10,000 euros, or about $11,500.

Mr. Hemmati also announced that the government would subsidize medicine and food to ensure that essential goods could be bought. Mr. Rouhani said on Monday that 1,500 items would be banned from import to save foreign currency for more pressing items. And the central banker lifted a ban on currency trading that some economists say has worsened the crisis.