Yet the most vitriolic remarks at the rally, which occurred Saturday, came from the American delegation, which included former governors Bill Richardson and Tom Ridge, former George W. Bush administration U.N. Ambassador John R. Bolton — and included a presentation of video statements from several current members of Congress.

Mr. Gingrich stoked the crowd by lamenting the Obama administration-backed nuclear deal. The accord went into effect a year ago this week and saw many economic sanctions on Iran lifted in exchange for an agreement by Tehranto curtail its long-disputed nuclear program.

The “dictatorship” in Iran “cannot be trusted,” Mr. Gingrich said, adding that “the agreement made with it is insane.”

Mr. Bolton went even further. “There is only one answer here: to support legitimate opposition groups that favor overthrowing the military theocratic dictatorship in Tehran,” he said. “Let me be very clear: It should be the declared policy of the United States of America and all its friends to do just that at the earliest opportunity.”

Both men appeared on stage with Mr. Richardson, a Democrat, who said he was “proud to be here with Speaker Gingrich and Ambassador Bolton,” and told the crowd that American “Democrats and Republicans are together here fighting with you.”

All three were flanked by multiple movie screen-size displays carrying the slogan: “Free Iran. Our Pledge: Regime Change.”

Saturday’s rally was a marathon that included more than nine hours of speeches and musical performances. The National Council of Resistance of Iran has come to be known during more recent years as perhaps the only dissent group on the planet with enough money and political juice to rally tens of thousands of supporters in the heart of Europe each June behind a collective call for the overthrow of Iran’s Shiite Islamist government.

No one disputes that the National Council has influence — some even describe it as the largest Iranian dissident group in the world. But the organization’s persistence and tactics have given it a double-edged reputation even among some of Iran’s Western critics.

National Council leader Maryam Rajavi headlined Saturday’s rally with a demand that Washington abandon the Iranian nuclear accord and take a far more aggressive posture toward Tehran.

Mrs. Rajavi has led the movement since its founder — her husband, Massoud Rajavi — went into hiding in 2003. In an email interview ahead of the rally, she said participants “represent the voice of millions of Iranians who are being oppressed in their country and who seek regime change and the establishment of a democratic, pluralist and non-nuclear government based on the separation of religion and state.”

“Their expectation of the next U.S. president, as with other Western leaders, is to abandon the policy of appeasement, which emboldens the Tehran regime to intensify the suppression of the Iranian people while continuing the policy of exporting terrorism to the region,” she said.

She also referred broadly to a “resistance” movement that she claimed has grown inside Iran during recent years even as the government has cracked down on opposition.

“Despite the intensification of the suppression over the past couple of years, we have witnessed a growing interest among the Iranian people, especially women and youth, toward the Iranian Resistance,” Mrs. Rajavi said. “The opposition to the regime is expanding. The Resistance’s network inside Iran is much more active in terms of organizing strikes, protests, sit-ins and other protest acts inside the country and even inside prisons. The Resistance has had numerous achievements in this regard.”

She also pushed back against characterizations of the National Council and its various affiliate organizations as acting like a cult.

“The source of these allegations is the Iranian regime’s intelligence ministry,” she said. “The regime’s lobbies in the West paint the Iranian opposition as a ‘cult’ or ‘terrorist group’ lacking popular support. By doing so, they want to perpetuate the notion that there is no other alternative for Iran except dealing with the ruling religious dictatorship.

“I have said repeatedly that we are not fighting to obtain power in Iran,” Mrs. Rajavi said. “We are not even fighting for a share of power. We are fighting to create a situation where the people of Iran are able to freely elect the people they choose. I and our movement will certainly support anyone who is elected through the ballot box in the course of free and fair elections monitored internationally.”

On a separate front, the National Council leader said that state political freedom and human rights have only worsened in Iran since the inking of last year’s nuclear deal.

“The pace of executions has intensified,” she said.

While the Obama administration lifted many economic sanctions on Iran under last year’s nuclear accord, the State Department has continued to list the nation as a state sponsor of terrorism, and international sanctions remain on the Islamic Revolutionary Guards Corps (IRGC).

But Mrs. Rajavi suggested the remaining sanctions were irrelevant.

She criticized, for instance, the prospective agreement that has made headlines recently between Boeing and the government in Tehran, asserting that if the deal goes through, planes made by the American aerospace giant “will directly or indirectly be used by the IRGC” and “will facilitate the regime’s activities for sending forces and arms to Syria and other countries in the region.”

• This article is excerpted from a staff report that appeared in The Washington Times on July 11, 2016.