Tehran under pressure to issue statement on 35-year-old blogger amid reports of his death under interrogation

The family of an Iranian blogger taken into custody accused of opposition activism on Facebook fears that he has died under torture.

Police picked up Sattar Beheshti, 35, from his home in the city of Robat-Karim in the southwest of Tehran last week.

His relatives said on Wednesday they had received phone calls from the prison authorities asking them to collect Beheshti's dead body from the notorious Kahrizak detention center on Thursday.

Beheshti's alleged death cannot be independently confirmed but Baztab, a news website close to Mohsen Rezaei, a senior politician, reported that the blogger has lost his life during interrogations.

"Sattar Beheshti, who was arrested by Fata [cyber] police, has died while being interrogated," Baztab reported. Iran is recently reported to have arrested a number of Facebook activists. Although Facebook is blocked in the country, millions of Iranians access it through proxy websites or virtual private networks.

Sahamnews, a website close to the opposition leader, Mehdi Karroubi, said Beheshti had died "under torture" during an interrogation session with security officials.

"They called us today and asked us to collect his dead body tomorrow from Kahrizak," a family member told Sahamnews.

Kahrizak is a detention centre where Iran imprisoned many of the opposition activists caught up in the protests that followed the country's disputed presidential elections in 2009.

Before his arrest, Beheshti wrote in his blog: "They threatened me yesterday that my mother would wear black because I don't shut my mouth."

Speaking to Masih Alinejad, a UK-based Iranian journalist, Beheshti's sister said: "Last Tuesday they raided into our house and took my brother with them … Today they called my husband and asked him to prepare me and my mother and buy a tomb for his dead body."

Commenting on reports about Beheshti's death, the UK's minister for the Middle East and North Africa, Alistair Burt, said: "I am shocked at reports that Sattar Beheshti, a young Iranian citizen, may have died in detention in Iran. Beheshti's only crime appears to be advocating the defence of human rights on the internet.

"Tragically, we have seen many similar cases of Iranians being locked up and mistreated in prison for expressing such views. If these reports are true, this is yet another disgraceful attempt by the Iranian government to crush any form of free expression by its citizens. The Iranian authorities have full responsibility for Beheshti's welfare in prison and I call on Iran urgently to confirm what has happened to him."

Many protesters are believed to have been tortured to death in Kahrizak, and several claim to have been raped. An Iranian doctor who examined the victims of Kahrizak was shot dead in September 2010.

Kahrizak became a scandal for the regime when Mohsen Rouholamini, the son of a former senior adviser to the Revolutionary Guards, was named among prisoners who had died at the centre.