Reformists in Iran are under pressure, detainees face torture and abuse, and people are being executed at an "alarming" rate, a UN monitor studying human rights in the tightly controlled country says.

The bleak picture presented to the UN Human Rights Council on March 13 comes ahead of a May 19 presidential election in Iran.

"All reports indicate a high level of control over citizens and that democratic space is severely limited," Asma Jahangir, the UN special rapporteur for Iran, told the council in Geneva.

Jahangir did not refer directly to the election, but she noted that three opposition figures who publicly challenged the official results of Iran's 2009 presidential election -- former Prime Minister Mir Hossein Musavi; his wife, university professor Zahra Rahnavard; and reformist cleric Mehdi Karrubi -- have been kept under house arrest for nearly six years without being formally charged.