Iran has announced the first execution of a protester convicted over the recent anti-government unrest.
Mohsen Shekari was hanged on Thursday morning after being found guilty by a Revolutionary Court of "moharebeh" (enmity against God), state media said.
He was accused of being a "rioter" who blocked a main road in Tehran in September and wounded a member of a paramilitary force with a machete.
An activist said he was convicted after a "show trial without any due process".
"The international community must immediately and strongly react to this execution," Mahmood Amiry-Moghaddam, director of the Norway-based group Iran Human Rights, said in a statement.
"If Mohsen Shekari's execution is not met with serious consequences for the government, we will face mass execution of protesters," he added.
The judiciary's Mizan news agency reported that a Revolutionary Court was told that Mohsen Shekari had blocked Tehran's Sattar Khan Street on 25 September and used a machete to attack a member of the Basij Resistance Force, a volunteer paramilitary force often deployed to quell protests.
On 1 November, the court found Shekari guilty of fighting and drawing a weapon "with the intention of killing, causing terror and disturbing the order and security of society" and convicted him of "enmity against God", Mizan said.
He appealed against the verdict, but it was upheld by the supreme court on 20 November, it added.
Iran Human Rights said Shekari was "denied access to his lawyer throughout the interrogation phase [and] legal proceedings".
It also said the hard-line Fars news agency aired his "forced confessions" hours after his execution. In the video, a bruise on his right cheek is visible.
Opposition activist collective 1500tasvir tweeted: "While his family were still hoping for an appeal and had no news from the case, the Islamic Republic unexpectedly executed him."
The judiciary has so far announced that at least 11 other people have been sentenced to death by Revolutionary Courts on the charges of "enmity against God" or "corruption on Earth" in connection with the protests. The defendants' identities have not been disclosed.
Amnesty International has said the courts operate "under the influence of security and intelligence forces to impose harsh sentences following grossly unfair trials marked by summary and predominantly secret processes".
'A huge gamble'
Analysis by Parham Ghobadi, BBC Persian
Mohsen Shekari's arrest, trial and execution took less than two and a half months.
A rushed execution of a young protester might deter others from taking to the streets. However, it might prove to be a double-edged sword for the Iranian regime, which is seeking to instil fear but causing anger.
Protesters have proven over and over again that they no longer have any fear. The funeral of each one killed by security forces has turned into an anti-government demonstration.
The hanging is therefore yet another huge gamble for the regime and one that might give fresh impetus to the protests on the streets.
Amnesty International said Shekari's execution "[exposed] the inhumanity of Iran's so-called justice system as dozens of others face the same fate".
"The international community must urgently call on the Iranian authorities to immediately halt all planned executions and stop using the death penalty as a tool of political repression against protesters in their desperate attempt to end the popular uprising," it added.
UK Foreign Secretary James Cleverly tweeted that the world "cannot turn a blind eye to the abhorrent violence committed by the Iranian regime against its own people", while German Foreign Annalena Baerbock lamented that "the Iranian regime's contempt for humanity knows no bounds".
Judiciary chief Gholamhossein Mohseni Ejei has said that he has instructed judges to "avoid showing unnecessary sympathy to main elements of these riots and issue tough sentences for them" as a deterrent.
A majority of the members of Iran's parliament have also demanded "decisive action" from the judiciary and "qisas", or retaliation in kind, for those who have "waged war" on the state.
Iran is second only to China in the number of executions carried out annually.
Even before the current unrest there had been what UN human rights chief Volker Türk has called an "alarming increase" in executions in Iran, with the number reportedly passing 400 for the year for the first time since 2017.
The women-led protests against Iran's clerical establishment were sparked by the death in custody of Mahsa Amini, a 22-year-old woman who was detained by morality police on 13 September for allegedly wearing her hijab, or headscarf, "improperly".
They have spread to 160 cities in all 31 provinces and are seen as one of the most serious challenges to the Islamic Republic since the 1979 revolution.
Iran's leaders have portrayed the protests as "riots" instigated by the country's foreign enemies. However, the overwhelming majority of protesters have been unarmed and peaceful.
So far, at least 475 protesters have been killed by security forces and 18,240 others have been detained, according to the Human Rights Activists' News Agency (HRANA). It has also reported the deaths of 61 security personnel.
Iran's Supreme National Security Council has said that more than 200 people have been killed, including members of the security forces.