US First Lady Laura Bush (Photo AP)

The United Nations special investigator on human rights in Burma is asking authorities there to free all the people who were arrested in a rare string of anti-government protests.

Paulo Sergio Pinheiro also voiced concern Friday about a hunger strike involving a group of detainees and allegations that detainees have been severely beaten and tortured.

Burma is experiencing its most sustained demonstrations in decades.

For more than a week, people have been taking to the streets to protest the government's sharp increase in fuel prices.

Security forces say they have detained about 50 people, but activists say more than 100 have been arrested.

Earlier Friday, U.S. First Lady Laura Bush called U.N. Secretary-General Ban Ki-moon to express her deep concern over the deteriorating situation in Burma.

She urged him to join the United States in condemning the recent arrests.

Burmese authorities have increased efforts to find other activists involved in the protests, and undercover police have maintained a strong presence in Rangoon since protests began.

Burma's government has been widely criticized for human rights violations, including the extended detention of opposition leader Aung San Suu Kyi and more than 12-hundred other political prisoners.