It was 7am when Delara Darabi phoned home. "Oh mother, I see the hangman's noose in front of me," she garbled. "They are going to execute me. Please save me." Moments later a prison official snatched the handset away. "We will easily execute your daughter and there's nothing you can do about it," he barked at the parents. Then, with a chilling click, the line went dead.

The desperate couple rushed to the Central Prison in Rasht, Iran, wailing at the guards to let them see their 22-year-old. As they prostrated themselves, an ambulance emerged, most probably with Delara's corpse inside.

"They took Delara to the gallows with nobody around her," Mohammad Mostafaei, one of her lawyers, said in a letter distributed to human rights groups. "They put the rope on her delicate neck. I do not know who the cruel person was to pull the chair from under her feet."

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Ms Darabi – dubbed The Prisoner of Colours for the love of painting she developed whilst on death row – was convicted for murdering her father's wealthy cousin in September 2003, when she was just 17. Although she initially confessed to the crime, she later said she had been persuaded to take the blame by her older boyfriend Amir Hossein. It was in fact Mr Hossein who had killed the rich relation, she said, to get the money.

The 19-year-old allegedly told Ms Darabi that she could save him from the gallows by confessing and that would be no risk to her own life because she was still a minor. The young woman complied. Her boyfriend was sentenced to 10 years in prison for complicity to murder; she was sentenced to death.

The execution, which happened on Friday, caught everyone by surprise. Not only had there been no formal notification 48 hours before the hanging, as required under Iranian law, but, just a fortnight earlier, Ms Darabi had actually been granted a two-month stay of execution by the head of the judiciary. The day before their daughter would end up being walked to the gallows, her parents had even visited her in jail where she had excitedly informed them there was to be an appeal so new evidence could be heard. Twenty-four hours later, she was dead.

Rights groups inside and outside Iran reacted with horror over the weekend as news of the secret hanging seeped out. "It appears Iran's head of judiciary has no ability to control his own judges," said Zama Coursen-Neff from the children's rights division of Human Rights Watch. "This is an outrageous violation of Iranian as well as international human rights law, and a callous affront to basic human dignity." Amnesty International said that the decision to rush the execution through in secret "appears to have been a cynical move on the part of the authorities to avoid domestic and international protests which might have saved Delara Darabi's life".

Iran leads the world in executing juvenile offenders, according to human rights groups, accounting for two-thirds of such deaths in the past four years. The hanging of Ms Darabi was the second known execution of a juvenile offender this year and lawyers in Tehran estimate that at least 130 more are waiting on death row.

It was the fate to which these young individuals were doomed that Ms Darabi sought to highlight through her haunting paintings. "Delara is not alone," she wrote to the president of Stop Child Executions. "Delaras are trapped in prisons and in need ... of defenders of human rights and humanity."

Many of her images are monochrome, the harsh charcoal lines depicting anguished, tortured faces. Others incorporate disturbing splashes of red, spattering the white headscarves of female prisoners, or washed across the background to suggest the hell of incarceration.

Those campaigning to free Ms Darabi put the artwork on display in Tehran. "I try to defend myself using colours, forms and words. These paintings are my swear to what I have not done," the prisoner wrote in the exhibition's blurb. "From behind the walls, I say hello to you, who has come to see my paintings."

As her family buried her at the weekend and the EU joined the chorus of criticism against the Iranian authorities, the human rights lawyer, Mr Mostafaei, recalled the personal gift Ms Darabi had bestowed on him. "She painted a picture of an old man playing the violin," he said. "I did not know that he was playing her death song."