But when she died suddenly in the custody of Iran's morals and virtues police - an organisation empowered by the supreme leader, Ayatollah Ali Khamenei, to enforce Islamic behavioural standards - officials reported it as suicide.

Now Baniyaghoub's family are insisting her death was suspicious and have engaged the country's most famous human rights lawyer, Shirin Ebadi, a Nobel peace prize winner, in an effort to prove she was murdered.

Baniyaghoub's ordeal started on the morning of October 12 while sitting with her fiancé, Hamid Chitsaz, in a park in the western city of Hamedan. Officers arrested the couple because they were not legally related and not entitled to be alone together under Islamic law.

Such treatment must have been galling for such a highly qualified couple. Baniyaghoub, 27, a graduate of Tehran University's elite medical school, was a GP in a remote village in one of Iran's poorest regions and had ambitions to qualify as a heart specialist or urologist. Her fiancé worked as a radio presenter with the state broadcaster IRIB. Nevertheless, they were sent before an Islamic judge who ordered their detention.

Chitsaz, who has since lost his job over the matter, was released after his family paid bail. But Baniyaghoub was detained overnight following delays in informing her parents, who live in Tehran, more than 200 miles away. By the time her father, Abolghasem, 61 - a retired clerical worker with the revolutionary guards - arrived to secure her release, Baniyaghoub was dead. A medical report stated she was strangled.

Officials claimed Baniyaghoub, a devout Muslim who prayed every day, was consumed by shame over her crime and used a chair and a piece of textile to hang herself from a beam in her cell.

Baniyaghoub's family say she would never have taken her own life, not least because of her strong Islamic faith, which taught her to regard suicide as a sin. Moreover, they argue that she had a legal right to be with Chitsaz because the couple had undergone sigheh, a Shia custom of temporary marriage recognised by the Iranian authorities. She would therefore have felt no shame in their relationship.

Baniyaghoub's parents believe she died after being assaulted by one of her captors, and are calling for her corpse to be re-examined. One theory is that she became involved in an argument with officers after her brother, Rahim, told her in a phone call that her father was on his way to collect her. As evidence, they say blood started flowing from the nose and ears of her corpse while it was being prepared for burial, suggesting possible brain injuries. The time of Baniyaghoub's death was recorded as 9pm, less than 20 minutes after she spoke to her brother.

'Zahra's voice was calm when she spoke to her brother. She wasn't suicidal,' a relative, who asked to remain anonymous, told The Observer. 'The family believe that, as a result of the phone call, she felt some mental support and told these guys they would find out who they were dealing with when her father arrived. When we examined her body, we found bruises on her legs. We think they started kicking and beating her and that she has died after her head knocked against something. Zahra loved serving and saving people. She was also deeply in love with her fiancé. Why would she have committed suicide?'

When her father turned up, he was not initially told of her death but instructed to return the next day, when he was finally informed. 'They humiliated Zahra's father when he first arrived, telling him to wait in the street and saying, "Your daughter is not morally fit to be a doctor." They did all this when she was already dead,' the relative said.

A detention centre report is said to contain a spate of inaccuracies, including a deliberate alteration to the date of Baniyaghoub's arrest to make it coincide with that of her death. Iranian law prohibits the detention of suspected morals offenders for more than 24 hours.

Ebadi is calling on Iran's judiciary chief, Ayatollah Mahmoud Hashemi Shahroudi, to open an investigation and supervise it from Tehran because of a lack of co-operation from the authorities in Hamedan. The call has been supported by a group of former MPs. Prompted by the record of Baniyaghoub's father as a former employee, senior revolutionary guard commanders have weighed in by asking Khamenei's office to investigate.

Ebadi said the case raised disturbing questions about Iran's treatment of women: 'How can it be that a 27-year-old woman, a fully qualified doctor, doesn't have the right to talk to her fiancé in a public park and the morals police have the right to arrest her?'

She compared Beniyaghoub's death to that of Zahra Kazemi, an Iranian-Canadian photojournalist who died in Tehran's Evin prison four years ago, apparently after suffering a brutal beating by interrogators. The Supreme Court last week ordered a new investigation into Kazemi's death, citing 'flaws' in an original investigation.