"What happened was that twice some delegations in these conversations complained about the presence of some demonstrators close to the premises of the conference," said EU diplomat Enrique Mora, who is coordinating the talks between Iran, Britain, China, France, Germany and Russia.

"They complained in particular about the use by those demonstrators of loud-speakers at full power, the whole day, with the music, the slogans, whatever," he added.

"On two occasions, I reported to the Austrian authorities these complaints. That is exactly my duty as coordinator of the talks."

Vienna police asked the protesters to move from outside the Grand Hotel where talks resumed on Saturday.

But Mora insisted he was "not involved" in that request.

"I was never informed nor consulted on the measures (of the police), I never said what they should do. We respect the right of demonstration as a basic right of speech," the diplomat said.

The exiled opposition group, the National Council of Resistance of Iran, slammed the police action as "a violation of the freedom of expression", but said the protesters had agreed to move nonetheless.

"We have one enemy -- the Iranian regime -- and we did not attack anybody from the EU or the Austrian authorities," spokesman Ehsan Ayatollahi told AFP.

The goal of the Vienna talks is to find a way back to a nuclear accord, signed in 2015 and known as the Joint Comprehensive Plan of Action, under which Iran was offered relief from sanctions in return for guarantees that it will not seek to build an atomic weapon.

Former US president Donald Trump walked away from the JCPOA in 2018, but his successor Joe Biden wants to revive it.