The idea she might pardon her son's killer first came to Samereh Alinejad in a dream. It was a message she didn't want to hear.

Abdollah Hosseinzadeh was stabbed and killed in a street brawl in the autumn of 2007 when he was only 18. He had known his killer, Balal. The two, barely out of their teens at the time, had played football together. Abdollah was the second son Alinejad had lost, her youngest died as a boy in a motorbike accident when he was 11. Furious in her grief, she was determined Balal would hang.

But as Balal's execution date drew nearer, Abdollah appeared to his mother in a series of vivid dreams.

"Ten days before the execution was due, I saw my son in a dream asking me not to take revenge, but I couldn't convince myself to forgive," she told the Guardian. "Two nights before that day, I saw him in the dream once again, but this time he refused to speak to me."

Speaking by phone from Iran's northern Mazandaran province, on the Caspian Sea, Alinejad said she had no intention of sparing Balal's life until the moment she asked for the noose to be removed from his neck. Her last-minute pardon was a remarkable act of humanity that moved hearts across Iran – and the world – but it took Alinejad by surprise as much as it did Balal, his relatives and her own family.

A stream of relatives, her brother and her mother, flowed through her house the night before the execution. Painfully aware of the grief she had carried in the seven years since her son was killed, none of them attempted to change her mind. "I stood very firm in my belief that I want him punished, so they didn't expect me to forgive."

As Abdollah's legal guardian, Alinejad's husband Abdolghani had the power under Iranian law to overturn the death penalty, but he had relinquished that responsibility to his wife.

"We couldn't sleep that night, we were all awake until morning. Until the last minute, I didn't want to forgive. I had told my husband just two days before that I can't forgive this man, but maybe there would be a possibility, but I couldn't persuade myself to forgive." Alinejad had been assured: "My husband said, look to God and let's see what happens."

In the early hours of last Tuesday, Alinejad was outside the gates of Nour prison, among the crowd gathered for Balal's execution.

Balal stands in the gallows during his would-be execution ceremony. Photograph: Arash Khamooshi/AFP/Getty Images

"You have the final say, my husband had said," she recalled. "He said you've suffered too much, we'll do as you say."

After recitation from the Qur'an was read, prison guards had hooked a rope around Balal's neck as he stood on a chair blindfolded, his hands tied behind his back. Iran's Islamic penal code allows the victim's heir – "walli-ye-dam" – to personally execute the condemned man as Qisas (retribution) – in this case by pushing away the chair he was standing on.

Seconds away from what could have been his final breath, Balal pleaded for his life and called out for mercy. "Please forgive," he shouted, "if only for my mum and dad," Alinejad recalled. "I was angry, I shouted back how can I forgive, did you show mercy to my son's mum and dad?"

Others in the crowd watching the scene in anguish also called out for the family to spare Balal's life. "Amoo Ghani (uncle Ghani), forgive," they shouted, calling the victim's father by his first name.

Balal's fate then took an unexpected turn. Alinejad clambered up on a stool and rather than pushing away his chair, slapped him across the face.

"After that, I felt as if rage vanished within my heart. I felt as if the blood in my veins began to flow again," she said. "I burst into tears and I called my husband and asked him to come up and remove the noose." Within seconds, as Abdolghani unhooked the rope from Balal's neck, he was declared pardoned.

Balal's mother Kobra, sobbing, reached across the fence separating the crowd from the execution site, and embraced Alinejad before reaching to kiss her feet – a gesture of respect and gratitude. "I didn't allow her to do that, I took her arm and made her stand up … she was just a mother like me, after all."

Arash Khamoushi, a photographer for Iranian news agency Isna, captured the extraordinary scene in a series of pictures that flooded internet sites, newspapers and television sets across the world. Among the most poignant images is of the mothers, facing each other for the first time, holding one another in their arms.

"She was extremely happy, it was as if someone had given her wings to fly," Alinejad said. Hours ulater, after sparing one woman's child, she went to visit her own son's grave.

Samereh Alinejad and her husband Abdolghani remove the noose from Balal. Photograph: Arash Khamoushi/AP

Abdollah was brought up in a religious family. Alinejad is a housewife, Abdolghani is a retired labourer who works as football coach in the local club where both Abdollah and Balal used to play. Having lost both their sons, the couple now have only their daughter. Balal remains in jail. A victim's family can only save a killer's life, they can't lift a jail sentence, which is at the discretion of the judiciary in Iran, which has the worst record for executions worldwide after China.

Alinejad has not spoken to Balal's family other than when they met at Nour prison. "I didn't utter a single word to them in all these years, nor complain directly about why their son killed mine," she said. "But they're in touch with our relatives.

"Balal was naive. He didn't want to kill, it wasn't in his nature, he was angry in seconds and had a knife in his hand."

Finding herself suddenly a figure of inspiration for people across the world, Alinejad has one lesson she hopes her tragedy will help others to learn: "For young people not to carry knives when they're going out. When they kill a person, they don't just kill that person, mums and dads die too as a result."

She is pleased, she said, so many people were happy with her decision: "I'm glad when people now call me their mum."

One week after pardoning Balal, Alinejad has found a peace lost since her son's death. "Losing a child is like losing a part of your body. All these years, I felt like a moving dead body," she said. "But now, I feel very calm, I feel I'm at peace. I feel that vengeance has left my heart."