Mr Son was jailed for posting an article online

Mr Son was jailed more than four years ago, after he translated an article on democracy and posted it on the internet.

The release was part of a general amnesty of more than 5,300 prisoners to mark National Day on 2 September.

The move follows diplomatic pressure from the US and other Western nations on Vietnamese human rights.

A series of crucial votes is due to take place in the US Congress on normalising trade ties with Vietnam, and President George W Bush is due to visit Vietnam in November.

Restrictions

Mr Son told the BBC he felt overwhelmed with emotion to be back at home with his family.

"Nothing can describe the feeling of freedom, and I understand that my freedom was granted thanks to the efforts of many democracy-loving people in Vietnam and abroad," he said.

"I will never quit my ideals and my principles."

Mr Son will serve out three years of "administrative surveillance", which could involve travel restrictions or house arrest.

The activist was arrested after he translated and published online an article from the US state department website entitled What is Democracy?

He was charged with spying and jailed for 13 years, but his sentence was later cut to five years.

Vietnam, which is seeking to join the World Trade Organisation, has come under sustained pressure to improve its record on human rights.

The US, European Union and human rights groups had all called for Mr Son's release.