Hundreds of protesters demonstrated Saturday in New York and pacifist groups took to the streets in dozens of other US and Canadian cities in a "Day of Mass Action" against a possible war with Iran.

About 500 protesters gathered in Manhattan's Times Square and marched to the headquarters of the US mission to the United Nations and to the Israeli consulate.

"No war, no sanctions, no intervention, no assassinations," read a banner leading the march.

The demonstrations came as Europe and the United States slapped tough new sanctions on Iran, and Israel this week launched new threats of military intervention if the Islamic republic fails to rein in its suspected nuclear development program.

There is heightened speculation that Israel is contemplating air strikes against Iranian nuclear facilities, fueled in part by US Defense Secretary Leon Panetta's comments to the Washington Post in which he said he believes there is a "strong possibility" that Israel will launch such air strikes this spring.

Iran admits it has a nuclear program but insists it is for peaceful purposes like generating electricity.

"The actions of the Iranian government in no way justify a US war on Iran," Debra Sweet, director of the organization "The World Can't Wait," told AFP at the New York march.

The protest joined efforts from a coalition of about 60 pacifist and human rights organizations.

A leaflet distributed at the New York demonstration said "in many ways, US war on Iran has already begun," citing as examples "harsh economic sanctions" against Tehran, "killing Iranian scientists in car bombings" and that "US aircraft carriers are right off Iran's shore."

"I don't know what (US President Barack) Obama will do but I do know what he has done, which is very hard sanctions that only will hurt ordinary people," Sweet said.

In Los Angeles, activists dressed themselves in orange prison jumpsuits and wore black hoods similar to Guantanamo detainees as part of the anti-war protest there.

Other peace marches were held Saturday in Boston, Chicago, Philadelphia, San Francisco, Washington and other US cities, according to organizers.

In Canada protests were staged in Calgary and Vancouver, and events were also planned in Britain, Ireland and India.