Pham Hong Son and Le Quoc Quan were detained on April 4 amid tight security put in place for the trial of Cu Huy Ha Vu, who was jailed for seven years in one of the communist nation's most politically charged cases in years.

The two men were freed on Wednesday evening, Le Quoc Quan told AFP by telephone, adding that the case against them had been dropped.

"We were arrested together, detained separately and released at the same time together."

Vietnam confirmed last week that the two prominent dissidents were detained "for causing public disorder" and were under investigation.

Diplomatic sources said earlier the pair were arrested while standing outside the court complex during Vu's trial.

Le Quoc Quan, a lawyer, said he and Son were considering taking legal action against the district authorities over their arrest and detention.

"It's not acceptable to both of us. We were arrested unlawfully," he said. "We protested peacefully."

Quan alleged he was roughed up by a policeman who hit him on the head during his first night in a detention cell.

The following day they were moved to a prison where "they treated us ok", he said.

He also said authorities broke the door of his office to search it after his arrest.

Quan was jailed for about three months in 2007. He was released following protests from the United States.

He had been arrested and accused of taking part in "activities to overthrow the people's government", shortly after returning from a five-month fellowship with the Washington-based National Endowment for Democracy (NED).

Son, a medical doctor, was earlier arrested in 2002, a few weeks after translating and publishing online an article entitled "What is democracy?" taken from the US State Department's website.

He spent more than four years in jail and a further three years under house arrest.

Vu, who was charged after twice trying to sue the prime minister, was sentenced last week for spreading anti-state propaganda that advocated an end to one-party communist rule.

His father was a member of revered founding president Ho Chi Minh's provisional cabinet from 1945.

Europe and the United States have expressing deep concern over the 53-year-old's conviction, saying is inconsistent with international human rights conventions to which Vietnam is a party. Hanoi denies the accusations.

Agence France Presse - April 15, 2011