(Reuters) - Iran is cracking down on activists and their lawyers, meting out harsh sentences in an effort to quash pro-democracy activities, United Nations human rights experts said on Friday.

In a joint statement, the independent experts called for the immediate release of human rights defenders including Narges Mohammadi, whom they said was rearrested on April 21 to serve a six- year prison sentence handed down by an appeals court.

"The conviction and extremely harsh sentencing of human rights defenders is an indication of mounting repression against the legitimate activities of human rights defenders and represents a serious setback for the protection of human rights in Iran," said Ahmed Shaheed, U.N. special rapporteur on Iran.

Thousands of opposition supporters have been detained since the disputed 2009 presidential election won by President Mahmoud Ahmadinejad, including scores of senior reformist figures.

Mohammadi, former vice-president of the Defenders of Human Rights Centre, founded by rights lawyer and Nobel peace laureate Shirin Ebadi, was convicted of "assembly and collusion against national security, membership in the Defenders of Human Rights Centre and propaganda against the regime," the statement said.

She is said to be in "extremely fragile" health, it said. A U.N. official told Reuters that Mohammadi is believed to be held in Tehran´s Evin Prison.

"Human rights defenders play a fundamental role in ensuring a democratic society which respects human rights. They must be allowed to carry out their work without facing intimidation, harassment, arrest and prosecution," said Margaret Sekaggya, special rapporteur on human rights defenders.

Lawyers representing activists in the Islamic republic are also facing difficulties as they are being identified with their clients´ causes, according to Gabriela Knaul, special rapporteur on the independence of judges and lawyers.

"The government has an obligation to ensure that lawyers can perform all of their professional functions without intimidation, hindrance, harassment or improper interference and that they do not suffer prosecution for any action taken while carrying out their duties," she said.

Abdolfattah Soltani and Nasrin Sotoudeh, both lawyers who have represented many high-profile political and human rights activists, are among those to have been jailed for carrying out their legitimate work, according to the statement.

Soltani, co-founder of the Defenders of Human Rights Centre, was arrested in September on charges of collusion, anti-regime propaganda and acquiring property through illegitimate means, leading to an 18- year prison sentence and 20-year ban on practicing law, it said.

Sotoudeh, a human rights lawyer, was arrested in September 2010 and sentenced to six years by an Iranian appeal court, along with a 10- year ban on her practicing law, it said.

Shaheed, a former foreign minister of the Maldives, and the other investigators report to the U.N. Human Rights Council.

Despite repeated requests, he has never been allowed into Iran, but the Geneva forum renewed his mandate in March for a second year after he reported a high rate of executions as well as abuse of minorities, and persecution of homosexuals and labor unions.

Mohammad Javad Larijani, head of Iran´s High Council for Human Rights, told the U.N. forum in March that his government had "repeatedly manifested its unwavering commitment toward the advancement of human rights" and that "self-monitoring" was a key principle. The aim was to build a prosperous society based on justice, equality, legitimate freedoms and development.

The report submitted by Shaheed "amounted to nothing more than the repetition of a barrage of unsubstantiated and biased contentions traditionally levied" against Iran, Larijani said. (Reporting by Stephanie Nebehay; Editing by Myra MacDonald) (© Thomson Reuters 2012. 05/04/12)