A prominent Vietnamese dissident, Nguyen Dan Que, has again been taken into custody, the government said on Thursday.
A foreign ministry spokesman said Dr Que was arrested on Monday at his home in Ho Chi Minh City.
The official said Dr Que had been "caught red-handed" breaking Vietnamese law, but gave no details of the alleged offence.
Dr Que, 61, has already spent over eighteen years in prison for advocating improvements in democracy and human rights in Vietnam.
The human rights group Amnesty International demanded his immediate release.
The arrest of this well-respected academic and winner of numerous human rights awards is certain to be met with international outrage
"This is yet more evidence of the ongoing harsh crackdown on freedom of expression by the Vietnamese authorities," Amnesty said.
Vietnamese authorities denied that Dr Que was arrested for political reasons.
"There is nobody arrested in Vietnam for the reason of opinions," Foreign Ministry spokeswoman Phan Thuy Than said.
Dr Que, an endocrinologist, was detained without trial between 1978 and 1988, after he criticised national health care policy.
After his release he set up a democratic rights movement, but was arrested in 1990 and sentenced to 20 years imprisonment.
He was released under an amnesty five years ago.
Over the last year several people have been detained or placed under house arrest by the authorities.
In November 2002, Le Chi Quang was sentenced to four years in prison after criticising the government on email.
And Nguyen Khac Toan, a former soldier and businessman, was sentenced to 12 years in prison in December 2002 on charges of spying.
There are at last six other leading dissidents awaiting trial in Vietnam's prisons.