The head of Iran’s Reformists Front, the organisation instrumental in securing the election of the country’s president, Masoud Pezeshkian, has been arrested by the Islamic Revolutionary Guards Corps (IRGC) in a move that is likely to exacerbate tensions over the handling of recent street protests.

Azar Mansouri, the secretary general of the Islamic Iran People party, had expressed deep sorrow at protesters’ deaths, and said nothing could justify such a catastrophe. She had not in public called for the supreme leader, Ali Khamenei, to resign.

In what looked like a decisive roundup of the key reformist figures outside government, Ebrahim Asgharzadeh, the head of the front’s political committee, and Mohsen Aminzadeh, a deputy foreign minister under the former president Mohammad Khatami, were also arrested.

Further arrests were made on Monday including of Hossein Karroubi, the son of Mehdi Karroubi, a past reformist presidential candidate who spent years under house arrest. Mehdi Karroubi had said Iran’s plight was the direct result of Khamenei’s destructive domestic and international politics, but the IRGC has alleged his son was the “inciter, compiler, and publisher of this destabilising statement”.

At least two other prominent figures in the Reformists Front, an umbrella group of as many as 27 reformist factions, have been told to appear at the prosecutor’s office in Evin jail on Tuesday. The moves seem designed to prevent the spread of criticism of the way the security services handled the protests.

The official government death toll is 3,000, but others put the figure substantially higher.

In a call for Mansouri to be released, the National Unity party condemned what it described as “security confrontations with well-known peaceful and non-violent forces that had pursued their activities within the framework of the law, especially at a time when the country faces external threats and serious internal challenges”.

The prosecutors’ office in Tehran claimed those arrested had made every effort to “justify the actions of the terrorists’ infantry”, and stated they were acting in league with the US and Israel. They were also accused of “targeting national unity, taking a stance against the constitution, promoting surrender, perverting political groups and creating secret subversive mechanisms”.

Justifying the unprecedented crackdown, Gholam-Hossein Mohseni-Ejei, the head of the judiciary, said: “Those who issue statements against the Islamic Republic from within are in agreement with the Zionist regime and America.” He described the people who issued the statement as “wretched and miserable” and threatened they would “suffer losses”.

In a statement last week, Mansouri said: “We will not allow the blood of these dear ones to be consigned to oblivion or the truth to be lost in the dust. Pursuing your rights and striving to clarify the truth is the human duty of us all. And with all our being, we declare our disgust and anger toward those who, ruthlessly and recklessly, dragged the youth of this land into earth and blood.

“No power, no justification and no time can sanitise this great catastrophe,” she added. Mansouri has not supported foreign interventions.

Her seizure follows the arrest of four other Iranian human-rights defenders who had signed a statement backed by 17 prominent activists demanding a “free, transparent referendum” to establish a new, democratic government in Iran. Three signatories were initially arrested – Vida Rabbani, Abdollah Momeni and Mehdi Mahmoudian – but it appeared a fourth signatory, Dr Ghorban Behzadian-Nejad, a senior adviser to Mir-Hossein Mousavi, the reformist prime minister, was also arrested.

The statement from the 17 read: “The mass killing of justice seekers who courageously protested this illegitimate system was an organised state crime against humanity.” The arrests come as Iranian leaders maintain a hard line ahead of expected talks on its nuclear programme.

Separately, Narges Mohammadi, the Nobel peaceprize winner arrested 59 days ago, was given a new seven-year jail sentence.

She was briefly allowed to speak to her lawyer, Mostafa Nili, on Sunday for the first time. She revealed she had been transferred to hospital but had been then sent back to jail before her treatment was complete.

Pezeshkian has set up an inquiry into the protests, but it is unlikely to be critical of the IRGC, and the arrests of his former supporters show how little influence he wields over the key decision-makers in the government, a point that will be underlined if he remains silent about the arrest of his backers.

On a turnout of 49.7%, Pezeshkian won the presidency in June 2024 with 16.4m votes, decisively defeating his rival, Saeed Jalili, with 13.5m votes, yet Pezeshkian has struggled to use his mandate.

The Israeli prime minister, Benjamin Netanyahu, has brought forward a trip to Washington to lobby President Trump to include Iran’s missile programme in the talks under way between the US and Iran.

The talks in Muscat mediated by Oman that started on Friday are due to recommence this week, and Trump has said he is willing for the talks to focus solely on curtailing Iran’s nuclear programme, a position that alarms the Israelis and some in the Republican party.