Protesters from coast to coast called for an immediate end to the operations, which began on February 28 with joint US and Israeli strikes targeting Iranian military sites, nuclear facilities, leadership, as well as civilian infrastructure.

In New York City's Manhattan, hundreds gathered at Union Square on Saturday afternoon.

They chanted slogans including "Biggest Threat in the World Today: Donald Trump and the USA" and "Trump Says More War, We Say No More."

Demonstrators carried signs reading "Hands off Iran," "No New US War in the Middle East," and "Money for People's Needs, Not War with Iran."

"This is an unjust war. This is another regime-change war, which usually comes from obviously corrupt reasons like land expansion -- basically colonizing other countries, taking advantage of them," Maya, one of the demonstrators in the square, told Xinhua.

"From what I saw, we're spending a billion (US) dollars a day on this war while we're in an affordability crisis. So obviously that money should be used for US citizens. It can be used for health care, for better child care, public transit -- anything except for this war," she added.

"I think that this war is terrible," said Maggie Morales, a New Yorker. "We know that this is something that only benefits him (President Donald Trump) personally. It does not benefit our country. It does not benefit our people. Unfortunately, it puts us more in danger."

"We already have so many stories, so much history of going to other countries and hurting other people with no benefit for the United States. And it's terrible that we can't learn from past lessons," Morales said.

"Today I didn't come for the protest," she added. "But I saw it, and I felt that I had to come and support them."

Speakers at the rallies condemned the strikes as violations of international law and accused them of destabilizing the Middle East.

They urged Congress to act swiftly to halt the operations.

"We've already seen that our current system is incapable of stopping the continued onslaught and bloodshed. We saw how Congress, both Democrats and Republicans, were unable to pass a War Powers resolution that would have stopped this war. Shame!" Andre Easton from the Bronx, New York City, said in his speech.

"It has always been, and it will only be up to the working people of the world to join together and oppose war here and anywhere else in the world," he added.

In Los Angeles, hundreds rallied outside City Hall on Saturday afternoon.

They waved Iranian flags and chanted slogans such as "Bombing Iran is a crime."

Several participants told Xinhua the conflict had shattered regional stability and inflicted severe suffering on Iranians.

"The war is a distraction from the Epstein files, and they're killing civilians needlessly," an anonymous protester from Los Angeles said, calling on people to speak out for peace and reject another endless war.

The United States and Israel initiated large-scale strikes against Iran on February 28, prompting Iranian missile and drone responses targeting Israel and US assets in the Middle East.

The escalation has intensified regional tensions and produced broader international repercussions.