TEHRAN, - Iran rejected on Sunday criticism by a U.S.-based rights group over a crackdown on women flouting the strict Islamic dress code, saying the country's efforts were aimed at "fighting morally corrupt people."

Human Rights Watch on Wednesday accused the Islamic republic of arbitrarily arresting both women and men under the banner of "countering immoral behaviour" and called for their immediate release.

"In Iran, the walls of homes are transparent and the halls of justice are opaque," Joe Stork, deputy director of the Middle East division of Human Rights Watch, said in a statement.

Asked about the criticism of the national crackdown launched last month, Iranian Foreign Ministry spokesman Mohammad Ali Hosseini told reporters he was surprised, adding:

"Officials are trying to provide security for the whole Iranian nation and fighting rebels, drug smugglers and morally corrupt people who are violating both Iranian citizens and their belongings."

Under sharia, Islamic law, imposed after Iran's 1979 revolution, women are obliged to cover their hair and wear long, loose-fitting clothes to disguise their figures.

Violators can be given lashes, fines and imprisonment.

Since Mahmoud Ahmadinejad won the presidency in 2005, promising a return to the values of the revolution, hardliners have pressed for tighter controls on "immoral behaviour".

In what has become a regular occurrence ahead of the warm summer months, police in April launched a campaign against the growing numbers of young women testing the limits of the law with shorter, brighter and skimpier clothing.

In the early days of the campaign, police said thousands of people had received warnings but it was not clear how many were detained. Human Rights Watch said "thousands" had been arrested.

Barbers have also been warned against offering "Western hair cuts" or plucking the eyebrows of their male customers, according to Iranian media.