A 43-year-old Iranian woman is facing death by stoning unless an international campaign launched by her children forces the authorities to quash what her lawyer calls a bogus conviction.

In a case that highlights the growing use of the death penalty in a country that has already executed more than 100 people this year, Sakineh Mohammadi Ashtiani was convicted in May 2006 of conducting an "illicit relationship outside marriage."

Sakineh already endured a sentence of 99 lashes, but her case was re-opened when a court in Tabriz suspected her of murdering her husband. She was acquitted, but the adultery charge was reviewed and a death penalty handed down on the basis of "judge's knowledge" – a loophole that allows for subjective judicial rulings where no conclusive evidence is present.

Speaking to the Guardian, her son Sajad, 22, and daughter Farideh, 17, say their mother has been unjustly accused and already punished for something she did not do.

"She's innocent, she's been there for five years for doing nothing", Sajad said. He described the imminent execution as barbaric. "Imagining her, bound inside a deep hole in the ground, stoned to death, has been a nightmare for me and my sister for all these years."

Under Iranian sharia law, the sentenced individual is buried up to the neck (or to the waist in the case of men), and those attending the public execution are called upon to throw stones. If the convicted person manages to free themselves from the hole, the death sentence is commuted.

Iran, embarrassed by the international attention over stonings, has rarely practiced it in public in recent years. But the country still executed 388 people last year – more than any other country in the world apart from China, according to Amnesty International. Most are hanged.

Tonight protesters gathered outside the Iranian embassy in London to demand Sakineh's release.

Five years ago when Sakineh was flogged , Sajad was 17 and present in the punishment room. "They lashed her just in front my eyes, this has been carved in my mind since then."

Mohammed Mostafaei, an acclaimed Iranian lawyer volunteered to represent her when her sentence was announced a few months ago. He wrote a public letter about her conviction shortly after. "This is an absolutely illegal sentence," he said. "Two of five judges who investigated Sakineh's case in Tabriz prison concluded that there's no forensic evidence of adultery.

"According to the law, death sentence and especially stoning needs explicit evidences and witnesses while in her case, surprisingly, the judge's knowledge was considered as enough," he said.

Mina Ahadi, a human rights activist in Germany who helped Sakineh's children to launch their campaign internationally has been in regular contact with Sajad and Farideh.

She said that after the campaign was launched last week, she received phone calls from the families of two other women kept in Tabriz prison, where Sakineh is, revealing that they are also convicted of adultery and sentenced to death by stoning. Azar Bagheri, 19, and Marian Ghorbanzadeh, 25, are their names, Ahadi disclosed.

"Azar was arrested when she was just 15. They couldn't punish her before she became 18 years old according to the law, so they waited until now … and want to stone her to death," Ahadi said. She has been subjected to mock stonings, complete with partial burial in the ground. "They're preparing her for the real one," said Ahadi.

Ahadi who has been following the stoning sentence in Iran over the past few years says that she is aware of the names of 12 other women who are sentenced to death by stoning in Iran at the moment.

"These are just the women I know, I estimate that at least 40 to 50 other women are waiting for the same destiny in Iran right now," she said.

"Stoning to death is not simply just a judicial punishment, it's a political means in the hands of the Iranian regime to threaten people. It has more function than just a simple punishment for them."