The German government is under intense political pressure to toughen its stance on Iran as opposition politicians demanded prosecution of a senior cleric who is visiting the country and Tehran's ambassador was confronted with allegations of a spy ring run by Tehran.

As Germany's Sigmar Gabriel and other European foreign ministers meet with Iran's Javad Zarif on Thursday in Brussels, there is a fierce controversy raging in Europe's largest nation over the deadly crackdown on protests that has killed 22.

Berlin revealed it had summoned Ali Majedi, Iran’s ambassador to deliver a reprimand against Tehran's spying on individuals and groups with close ties to Israel.

The move comes after Mustufa Haidar Syed-Naqfi, a Pakistani cultural representative, was convicted of gathering intelligence on Reinhold Robbe, the former head of the German-Israel Friendship Society, and an Israeli-French economics professor in Paris, for Iran’s elite Revolutionary Guards.

The Foreign Ministry announced the December 22 summons on Tuesday calling it a completely unacceptable breach of German law.

“Spying on people and institutions with special ties to the state of Israel on German soil is an egregious violation of German law,” a ministry official said.

EU has called Iran's Mohammad Javad Zarif to talks in Brussels on Thursday with his French, British and German counterparts in efforts to preserve the 2015 deal to curb Tehran's nuclear ambitions.

The meeting between Mr Zarif and the three European parties to the landmark 2015 agreement comes after Iran warned the world on Monday to prepare for the withdrawal of the United States.

Meanwhile German prosecutors have been urged to swoop on the cleric tipped to succeed Ayatollah Ali Khamenei who is in the country for medical treatment.

The Iraqi born Ayatollah Mahmood Hashemi Sharoudi, 67, was admitted in the International Neuroscience Institute (INI) in Hannover last week, but his stay at the centre had been kept secret until a group of Iranian exiles started to demonstrate outside it and demanded Shahroudi’s arrest for alleged human rights abuses while serving as the Head of Iran’s Judiciary from 1999-2009.

He is currently Chairman of the Expediency Council that resolves differences between the Majles and the Guardian Council, with close links to the Supreme Leader.

Upon taking up his job as Iran’s chief judge, Mr Shahroudi had made a fundamentalist declaration that Iran’s politics needed to be rebuilt. “I have inherited a destroyed building that needs to be rebuilt from the scratch,” he said.

However, his tenure in office coincided with the post-2009 election unrests across Iran, during which more than 120 Iranians were killed by security forces and thousands were jailed by hardline judges.

Almost all reformist dailies were also closed down by the order of the judiciary, after Ayatollah Khamenei called them “the enemy’s fifth column”.

With the news of Mr Shahroudi’s presence in Germany becoming public, the country’s Green Party politician Volker Beck filed a criminal complaint against him on Sunday, accusing him of “mass murder activity that could be prosecuted under German law covering crimes against humanity”.

“Germany should not be a sanctuary for such people, who in their country persecute people for political or religious reasons and threatens them with death. The Iranian regime persecutes women, homosexuals, ethnic minorities and atheists,” Mr Beck said.

Iran’ former deputy Foreign Minister Ahmad Azizi has harshly criticised the BBC Persian TV for reporting on the protests against Mr Shahroudi’s presence in Germany. It interviewed the German-Iranian Majid Samii, the founder of INI, about his role in facilitating the ayatollah’s treatment.

Prof Samii has come under pressure on social media to explain his close relationship with Iran’s top leadership, at a time when it faces widespread opposition by the Iranian people at home.

“I have no political motives in assisting Mr. Shahroudi and it would be wrong to question patients’ background prior to treating them”.

A former member of the Islamic Dawa Party of Iraq, Mr Shahroudi lived in exile in Iran during the reign of Saddam Hussein and is currently aligned with the Khamenei’s camp within Iran’s religious establishment.

“Shahroudi is the favourite of Khamenei to succeed him as it was him who gave Khamenei the religious credentials to become the Supreme Leader. In addition, Khamenei sees Shahroudi’s link to the Iraqi Shias as a means of continuation of Iran’s influence in the neighbouring country”, an Iranian opposition member told the National.

“However, he is not necessarily the choice of Iran’s other political factions”, he added.

Mr Shahroudi is most recognised in the Western human rights bodies for instituting Iran's 2002 moratorium on stoning to death punishment.

Iran's campaign to salvage the 2015 nuclear accord has attempted to wether the recent protest. It hosted former British Foreign Minister Jack Straw and other retired statesmen even as the recent unrest was unfolding on the streets. Mr Straw told the head of Iran's atomic organisation, Ali Akbar Salehi that the current uncertainties about the future of the nuclear deal is “due to Trump’s unstable mind”.

For his part, Salehi has criticised the UK government for “blocking the sale of 900 tons of natural uranium from Kazakhstan to Iran”.