Iran’s security forces are on high alert amid four months of anti-government protests over the death of Mahsa Amini, a 22-year-old woman detained by morality police for allegedly violating the country’s strict Islamic dress code.

The protests quickly escalated into calls for the overthrow of the theocracy, marking one of the greatest challenges it has faced in more than four decades.

Iran blames the US and other foreign powers for the unrest, without providing evidence. The demonstrators say they are fed up with social and political oppression, corruption and an economy weighed down by Western sanctions and mismanagement.

Iran has arrested a number of foreigners in recent years, accusing them of undermining state security and trying them in secret Revolutionary Courts convicted by rights groups. Critics accuse Iran of using the foreigners as hostages to extract concessions from the West, which Tehran denies.

UN rights experts on Tuesday condemned Iran’s detention of 41-year-old Belgian aid worker Olivier Vandecasteele, who was sentenced last week to more than 10 years in prison and 74 lashes on charges of espionage and money laundering.

“We believe that Mr. Vandecasteele has been arbitrarily deprived of his liberty and is subject to enforced disappearance during periods of detention,” they said in a statement. “His right to a fair trial before an independent and impartial tribunal has been violated. These are flagrant violations of Iran’s obligations under international law.”

They said Vandecasteele, who was arrested last February, is in solitary confinement and suffering from serious health problems.