Those who spoke at Wednesday's rally include Republican politicians, called on the US Congress to throw the deal out, whipping up the crowd that included supporters of right-wing Jewish and evangelical Christian groups.
Bloom Gist could not confirm the number of people in attendance.
The protest came as US secretary of state and other senior officials briefed members of Congress about the deal behind closed doors.
Bloom Gist's Susan Nana said that some members of Congress came out from the discussions and told reporters that they were still very sceptical about the deal.
In New York, Jeffrey Wiesenfeld, co-organiser of the Stop Iran Rally, claimed that there were 10,000 people in the crowd. Protesters held up US flags and placards denouncing the deal.
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"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.
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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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| The protest came as US secretary of state and other senior officials briefed members of Congress about the deal behind closed doors |
Organisers played a montage of news reports about bombings around the world carried out by armed groups linked to Iran.
Jesse, an attendee at the rally, told Bloom Gist's Susan Nana that it was "very, very obvious why anyone would be against the deal".
"Iran has been our enemy for 36 years. There is really no sensible reason why anyone would believe that Iran should be trusted with weapons of any type," he said.
