A prominent government critic, Dr Nguyen Quang A, was detained by police for 15 hours at Hanoi airport on Tuesday September 1 after he arrived back from a foreign trip.
He said the police wanted to find out where he went and who he met during a visit to the United States. He said he refused to answer their questions and accused them of kidnapping him.
It is the first time the veteran civil society activist has been detained in such a way, although he has been subject to harassment in the past by police and Communist Party agents.
His detention came the day before the staging of mass parades and celebrations in Hanoi to mark National Day, the 70th anniversary of Ho Chi Minh’s declaration of independence for Vietnam in 1945.
Dr A said the police told him he was being held for national security reasons and originally said his passport had expired even though it is valid for another five years.
They showed him photographs of his meetings with Vietnamese activists in the US, who the police described as hostile forces.
About 50 friends, family members and other supporters arrived at the airport during the day to try to secure his release.
They said they were attacked by plainclothes agents and some were hurt.
Dr A is well known as an activist with the Civil Society Forum, a grouping of intellectuals and writers, who back a democratic transition in Vietnam.
He and some of his associates are former members of the Communist Party and have connections within leadership circles as well as with other dissidents.
Dr A is an advocate of gradual reforms within the current system.
In a post on his Facebook page Dr A said one of the police interrogators forcibly seized his phone when he began communicating by video link with his son who was in the Arrivals Hall.
He said some of the police told him that he was not welcome in Vietnam because of his activities.