The first death sentence has been issued over the protests in Iran that have shaken the country's clerical leadership, the judiciary said.
Weeks of protests since the death of Mahsa Amini in the custody of the morality police, have been met with a harsh response from Iranian authorities that has led to thousands of arrests.
Norway-based rights organisation Iran Human Rights said at least 20 people were facing charges punishable with death.
The person sentenced to death in a Tehran court, who was not identified, was found guilty of “setting fire to a government building, disturbing public order, assembly and conspiracy to commit a crime against national security”, as well as being “an enemy of God” and of “corruption on earth”, the judiciary website Mizan Online reported on Sunday.
Another court in Tehran sentenced five others to prison terms of between five and 10 years for “gathering and conspiring to commit crimes against national security and disturbing public order”, Mizan said.
Earlier this month, 272 of Iran's 290 parliamentarians demanded that the judiciary apply the death penalty, in “an eye for an eye” retributive justice against those who “have harmed people's lives and property with bladed weapons and firearms”.
“We are very concerned that the death sentences may be carried out hastily,” Iran Human Rights director Mahmood Amiry-Moghaddam told AFP.
“The international community must send a strong warning to the Iranian authorities that implementation of the death sentence for protesters is not acceptable and will have heavy consequences.”
Mizan and other local media also said the judiciary had charged more than 750 people in three provinces for involvement in “recent riots”.
More than 2,000 people had already been charged, nearly half of them in the capital Tehran, since the demonstrations began, according to judiciary figures.
The purge has also led to the arrest of dozens of activists, journalists and lawyers whose continued detention has caused an international outcry.
Iranian authorities on Sunday transferred to hospital prominent dissident Hossein Ronaghi who was arrested in September and has been on hunger strike for more than 50 days, his brother said.
Mr Ronaghi was taken to Evin prison after his arrest on September 24. His family said he was at risk of dying because of a kidney condition and that both his legs had been broken in prison.
On Sunday, his brother said he was moved to the Dey general hospital in Tehran.
“Hossein was taken to one of the departments of the Dey hospital,” Hassan Ronaghi wrote. He said his parents had been prevented from seeing their son. “His life is in danger.”
Iran criticised a meeting between French President Emmanuel Macron and Iranian dissidents on Friday, calling his comments after the encounter “regrettable and shameful”.
Mr Macron met four prominent dissidents, all of them women, and afterwards spoke of his “respect and admiration in the context of the revolution they are leading”.
Masih Alinejad, the prominent US-based activist who has campaigned against the compulsory headscarf and was at the meeting, told AFP: “President Macron recognised the Iranian revolution and that's a truly historical decision.”
According to IHR, at least 326 people have been killed by the security forces' response to nationwide protests.
This figure includes at least 123 people killed in the province of Sistan-Baluchistan, on Iran's south-eastern border with Pakistan.
Most were killed on September 30, when security forces opened fire on protesters after Friday prayers in Zahedan, the capital of Sistan-Baluchistan — a massacre activists have dubbed “Bloody Friday”.
The September 30 protest was triggered by the alleged rape in custody of a 15-year-old girl by a police commander in the province's port city of Chabahar.
A delegation from Iran's supreme leader Ayatollah Ali Khamenei expressed sadness and promised solutions in a weekend visit to Zahedan, official media said.
The city's police chief and the head of a police station had already been dismissed, local officials said.