Anti-war activists organised demonstrations on Sunday in New York, Washington and other U.S. cities, with signs carrying messages such as "hands off Iran."
Meanwhile, some Iranians said they feared the prospect of an enlarged conflict involving the U.S.
"Our future is dark. We have nowhere to go - it's like living in a horror movie," Bita, 36, a teacher from the central city of Kashan, said before the phone line was cut.
Much of Tehran, a capital city of 10 million people, has emptied out, with residents fleeing to the countryside to escape Israeli bombardment.
Several killed
Iranian authorities say more than 400 people have been killed since Israel's attacks began, mostly civilians. Israel's bombardment has decimated much of Iran's military leadership with strikes targeted at bases and residential buildings where senior figures slept.
Speaking in Istanbul, Iranian Foreign Minister Abbas Araqchi said his country would consider all possible responses. There would be no return to diplomacy until it had retaliated, he said."
The U.S. showed they have no respect for international law. They only understand the language of threat and force," he said.
Trump, in a televised address, called the strikes "a spectacular military success" and boasted that Iran's key nuclear enrichment facilities had been "completely and totally obliterated."
The world braced on Sunday for Iran's response after the US attacked key Iranian nuclear sites, joining Israel in the biggest Western military action against the Islamic Republic since its 1979 revolution.
Iran vowed to defend itself a day after the US dropped 30,000-pound bunker-buster bombs onto the mountain above Iran's Fordow nuclear site while American leaders urged Tehran to stand down and pockets of anti-war protesters emerged in U.S. cities.
Iran and Israel continued to trade volleys of missile attacks. An Israeli military spokesperson said Israeli fighter jets had struck military targets in western Iran. Earlier, Iran fired missiles that wounded scores of people and flattened buildings in Tel Aviv.
Tehran has, so far, not followed through on its threats of retaliation against the United States - either by targeting U.S. bases or trying to choke off global oil supplies - but that may not hold.
Iran has been launching missiles back at Israel, killing at least 24 people over the past nine days.
Air raid sirens sounded across most of Israel on Sunday, sending millions of people to safe rooms.
In Tel Aviv, Aviad Chernovsky, 40, emerged from a bomb shelter to find his house had been destroyed in a direct hit. "It's not easy to live now in Israel (right now), but we are very strong," he said. "We know that we will win."