A second high-profile detainee, dissident Tran Huynh Duy Thuc, was also released, his friend and former human rights lawyer Le Cong Dinh said.

The release of Hong and Thuc comes as General Secretary To Lam, Vietnam's top leader, departed Saturday for the United States for a working visit and to attend the 79th Session of the United Nations General Assembly.

Last September Hong was sentenced to three years in prison for dodging $275,000 in taxes related to her environmental campaign group CHANGE.

She was one of five environmentalists jailed for tax evasion, in what activists have called a campaign to silence them.

"She has been fully pardoned," her husband Hoang Vinh Nam told AFP. "It was very sudden."

Nam said they had speculated whether there could be some positive news about Hong ahead of Lam's US visit, but they had no clear information until yesterday about her release from jail in Gia Lai, a mountainous province in Vietnam's Central Highlands.

"She took a bus home, it took her 12 hours to reach Ho Chi Minh City and I picked her up from the bus station at 5:00 am this morning (22:00 GMT Friday).

"It's just amazing. She's good, she's healthy and she's the same person she was when she went in (to prison)."

- 'Spirit is strong' -

Hong founded CHANGE to mobilise Vietnamese, particularly young people, to take action on pressing environmental issues, including climate change, the illegal wildlife trade and pollution.

But the communist government has shown little appetite for dissenting voices on environmental issues.

It also tolerates no opposition to its one-party rule and regularly jails critics.

Internet entrepreneur Thuc was sentenced to 16 years in jail in 2010 for trying to overthrow the regime.

Three others, including Dinh, were jailed alongside him in a case which sparked global concern.

Dinh told AFP that Thuc was released from prison in central Nghe An province on Friday and flew to Ho Chi Minh City, where he was held in a government builidng close to his house.

At 4:00 am on Saturday, "he called his sister... and asked all of us to come to the People's Committee to pick him up. We came and after half an hour they released him."

"He looked very thin, and his hair is white but his spirit is very strong," said Dinh, who was released from jail in 2013.

At their trial, they were found guilty of a well-organised non-violent campaign, in collusion with "overseas exile reactionary organisations", aimed at overturning the government with the help of the internet.

In the United States, General Secretary Lam will chair an event marking the 30th anniversary of the establishment of diplomatic relations and the first year of a comprehensive strategic partnership between Hanoi and Washington, state media said.