Vietnamese authorities have released a prominent pro-democracy activist who was jailed four years ago on charges of spying.

Pham Hong Son was freed from a prison in Vietnam's northern Thanh Hoa province Wednesday and was taken by police to his home in Hanoi, the capital.

Vietnam announced earlier this week it would release Pham as part of a presidential amnesty marking the country's 60th independence day on September 2.

In Son's case, however, officials say he will remain under police surveillance for a three-year probation period, and face unspecified travel restrictions. The 37-year-old physician's wife, Vu Thuy Ha, says she fears he will be subject to house arrest.

U.S.-based Human Rights Watch says the fundamental freedoms of most political prisoners in Vietnam are restricted long after they are released from prison. The group is concerned about the type of treatment Pham will face.

An HRW representative in Asia, Sophie Richardson, said Wednesday, "Like many others [in Vietnam], Pham Hong Son should never have been arrested in the first place."

Son was arrested in Hanoi in 2002, several weeks after he translated an article on democracy by the U.S. State Department and published the document on the Internet. He was sentenced to 13 years in prison in 2003, but the term was reduced to five years on appeal.

Some information for this report was provided by AFP, AP and Reuters.