More than 70 planned protests were organized by Code Pink and Act Now to Stop War and End Racism, a US-based antiwar coalition, along with other groups, AP reported.
From Tampa to Philadelphia and San Francisco to New York, protesters carried signs and chanted antiwar slogans.
Trump ordered Friday’s airstrike at Baghdad’s international airport that killed Lieutenant General Qassem Soleimani, the head of Iran’s elite Quds Force and mastermind of its regional security strategy.
Iran has vowed retribution, raising fears of an all-out war, but it’s unclear how or when a response might come.
Protest organizers said the Trump administration has essentially started a war with Iran by assassinating Soleimani.
Protesters in Washington held signs that read “No war or sanctions on Iran!” and “US troops out of Iraq!”
Speakers at the Washington event included actress and activist Jane Fonda, who last year was arrested at a climate change protest on the steps of the US Capitol.
“The younger people here should know that all of the wars fought since you were born have been fought over oil,” Fonda, 82, told the crowd, adding that “we can’t anymore lose lives and kill people and ruin an environment because of oil.”
“Going to a march doesn’t do a lot, but at least I can come out and say something: that I’m opposed to this stuff,” said protestor Steve Lane of Bethesda, Maryland. “And maybe if enough people do the same thing, he (Trump) will listen.”
In Miami, nearly 50 protesters gathered. Drivers heard people shouting, “No more drone murders,” “We want peace now” and “What do we want? Peace in Iran.”
A few hundred demonstrators gathered in Times Square on Saturday chanting “No justice, no peace, US out of the Middle East!”
In Minneapolis, protesters gathered near the University of Minnesota holding signs and chanting. Among them was Meredith Aby, a longtime leader of the local Anti-War Committee.
“We need to be pulling out of Iraq, not sending thousands more troops. We need to be trying to cool things down with Iran, not pouring gasoline on a fire,” Aby, 47, said.
Public opinion polls show Americans in general have been opposed to US military interventions overseas. A survey last year by the Chicago Council on Global Affairs found 27% of Americans believe military interventions make the United States safer, and nearly half said they make the country less safe.