Protesters poured into New York's Times Square on Wednesday to denounce the Iran nuclear deal as a threat to Israel and global security, demanding that Congress reject the agreement.
Speakers, including Republican politicians, called on Congress to throw it out, whipping up the crowd that included supporters of right-wing Jewish and evangelical Christian groups.
"We're here as Americans to speak with one voice to say stop Iran now, reject this deal," said George Pataki, the former three-term Republican governor of New York.
"This is a God awful deal, this must be rejected. Congress must do its job and stand up for the American people, stand up for our safety and say no to this Iranian deal," he said.
Jeffrey Wiesenfeld, co-organizer of the Stop Iran Rally, claimed that 10,000 people had turned out. Protesters held up US flags and placards denouncing the deal.
A spokesperson for the organizers said protesters had "packed the entire block" on both sides of Seventh Avenue.
The rally expressed support for Israeli Prime Minister Benjamin Netanyahu, whose criticism of the deal has strained relations with President Barack Obama.
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Recent polls have suggested that of the 79 percent of Americans who heard about the deal, 48 percent disapprove.
Organizers played a montage of news reports about bombings around the world carried out by extremist groups linked to Iran. "Iran has been killing Americans for 36 years," it said.
"Stop the deal."
Scholar and Democrat Alan Dershowitz, a Harvard law professor, appealed to fellow liberals to side with Republican opposition.
"It is a bad deal for Democrats. It is a bad deal for liberals. I am here opposing this deal as a liberal Democrat," he said.
He called the deal bad for America, bad for world peace and bad for the security of the Middle East.
The Republican-majority Congress has 60 days to review the deal.
The Congress can pass a motion of disapproval, but President Barack Obama can then veto that. An override of the veto requires two-thirds approval in both the House and Senate.