Shortly after the cease-fire announcement, a U.S. official said American military strikes against Iran had stopped. Iran’s foreign minister, Abbas Araghchi, said in a statement that the Iranians would “cease their defensive operation,” and that “for a period of two weeks, safe passage through the Strait of Hormuz will be possible via coordinating with Iran’s Armed Forces.”

The cease-fire buys both sides time to try to reach a longer-term end to the war, which began at the end of February with the United States and Israel subjecting Iran to a withering military assault.

Iran accepted Pakistan’s cease-fire proposal after frantic diplomatic efforts by Pakistan and last-minute intervention by China, a key ally, according to three Iranian officials. Iran’s national security council officially confirmed the agreement, casting it as a victory in which the United States accepted Iran’s terms.

Earlier, as the day wore on, it was not clear whether an off-ramp would emerge from the talks. It was not even clear if there were talks.

At one point, with Mr. Trump threatening devastating strikes on power plants, bridges and other critical infrastructure — a possible war crime under international law — Iran stopped engaging in the indirect negotiations.

“A whole civilization will die tonight, never to be brought back again,” Mr. Trump warned earlier in the day, though he said he hoped “maybe something revolutionarily wonderful can happen.”

In the hours before the 8 p.m. deadline, the United States and Israel stepped up their attacks on Iran. All the while, the Pakistanis were reported to be redoubling their efforts to get a cease-fire.

In urging Mr. Trump to hold off, the country’s prime minister, Shehbaz Sharif, said on social media that diplomatic efforts were “progressing steadily, strongly and powerfully with the potential to lead to substantive results.” He also asked Iran to open the waterway for two weeks.

Iran’s ambassador to Pakistan said in a social media post on Tuesday evening that diplomacy to stop the war had taken a “step forward” from a “critical, sensitive stage.”

But as night fell in the Middle East, Iranians were bracing for the possibility of more strikes. Some formed human chains along bridges and around power plants across the country, videos and photographs posted by state and other local media showed. It’s unclear whether the demonstrations were spontaneous or planned by the government.

Here’s what else we’re covering:

• Railways struck: The Israeli military said it had launched airstrikes on eight bridges across Iran, and warned Iranians not to ride railroads until 9 p.m. local time. Iranian state media reported that at least three people were killed when a railway bridge was hit in the central city of Kashan.

• Condemnation of Trump’s threat: Some commentators on the right are splitting from Mr. Trump, while some Republican lawmakers have expressed concern that the threat could cause the president to lose public support. Democrats forcefully condemned Mr. Trump, with a growing number calling for him to be removed from office.

• Death tolls: The Human Rights Activists News Agency said at least 1,665 civilians, including 244 children, had been killed in Iran as of Monday. Lebanon’s health ministry on Monday said more than 1,500 people had been killed in the latest fighting between Israel and Hezbollah. In attacks blamed on Iran, at least 50 people have been killed in Gulf nations. In Israel, at least 20 people had been killed as of Monday. The American death toll stands at 13 service members, with hundreds of others wounded.

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