Haleh Sahabi, 55, had been allowed out of prison to attend the funeral of Ezatollah Sahabi, a former Iranian MP and prominent dissident.

Her son, Yahya Shamekhi, said she had become upset after security officials tried to stop the ceremony and is thought to have had a heart attack.

The Iranian authorities have denied the reports.

Funerals involving other opposition figures have also been broken up.

‘Agitated atmosphere’

Mr Shamekhi described what had happened at Ezatollah Sahabi’s funeral.

“When we took the body of my grandfather out for the funeral ceremony, officials tried to stop the ceremony – that made the atmosphere very agitated,” he said.

“Finally they forcefully grabbed the body and took it away. Then my mother fell down and become unconscious. The doctor told us she died because of a heart attack.”

But Alireza Janeh, head of security at the Tehran governor’s office, said there were no clashes.

Mr Janeh told the semi-official Isna news agency that Mrs Sahabi had died of heart problems exacerbated by “stress and hot weather”.

The funeral was attended by at least 2,000 people in the Lavasan area of Tehran despite reports of a heavy security presence in the area.

A mourner told the BBC: “Following the uprisings in the Middle East, the Iranian government is very nervous about people’s gatherings for any reasons, particularly if it is related to an opposition figure.”

Mrs Sahabi, a member of the “Mothers for Peace” group and a campaigner for women’s rights, was one of a group of government critics who were arrested in front of the parliament during President Mahmoud Ahmadinejad’s swearing-in ceremony. She was sentenced to two years in prison.

Ezatollah Sahabi was a member of the interim government installed after the 1979 Islamic revolution in Iran, which resigned in protest over the takeover of the US embassy in Tehran.

He died of stroke in a Tehran hospital on Monday at the age of 81.