Iran midfielder Saeid Ezatolahi has said his heart is "on fire" after learning a childhood friend was killed celebrating the USA victory against Iran.
Ezatolahi said "humanity is dimming by day" and has not slept after learning of his friend's death following Iran's World Cup match on Tuesday.
"I wish we could always remain that age, without concern, without hate, without jealousy, without fighting to destroy each other," he said on Instagram on Wednesday, posting an old photo with his friend as children dressed in football gear.
"This is not what our youth deserve. This is not what my Iran deserves.
"Be sure that some day when the masks fall and the truth appears, they'll pay for your family's mournful hearts and your mother's grief."
While he did not name his friend, Iranians identified him as Mehran Samak, who was said to be killed by security forces in Anzali after honking his car horn to celebrate USA's triumph over Iran.
Iran has been rocked by months of protests in the biggest challenge to Tehran since the Islamic Revolution. Protesters celebrated the USA victory against Iran on Tuesday, with fireworks marking the occasion in the Kurdish west, where dissent has been fiercely suppressed.
At the World Cup, Qatari authorities and pro-government fans have reportedly harassed those backing the protests and prevented them from brandishing items with popular slogans.
Iran's national team have also come under fire from authorities after initially refusing to sing the national anthem before matches. The players have since backtracked on the move, after their families were threatened with torture if team members fail to "behave" in Doha, according to CNN.
Iranian footballer Voria Ghafouri, who chose not to go to the World Cup and is an outspoken government critic, was arrested on charges of "tarnishing the national team" and spreading propaganda, an allegation commonly used to justify the arrest of dissidents.
The government has denied claims of his release.
Mostly young Iranians have continue to protest against Tehran despite being gunned down in their hundreds. Some were shot dead for honking car horns at security forces, while others were killed while watching demonstrations from the rooftops of their homes.
Several have already been sentenced to death, many on charges of "corruption against Earth" in what have largely been denounced as sham trials.
On November 1, Ezatolahi posted an apparent message of support for protesters his Instagram page. "We are all Iranians at the end. In hard times, we should support our brothers and sisters. God return peace to my country and dear people," he said.