Nguyen Van Dai was detained on Wednesday while preparing to meet European Union (EU) delegates to the EU-Vietnam human rights talks in Hano.
Mr Dai, a well-known defender of human rights and religious freedom, is facing accusations of "spreading propaganda" under Article 88 of the Penal Code, according to Christian Solidarity Worldwide (CSW).
It was reported that Mr Dai was assaulted earlier this month after attending a conference organised to talk to citizens about their rights.
The discussion including educating people on their right to freedom of religion or belief.
The charge of spreading propaganda carries a long jail term.
Mr Dai has previously served a four year prison sentence after being found guilty under Article 88 for the same charge.
Additionally, Protestant pastor and activist Nguyen Trung Ton, a pro-democracy activist and religious freedom defender, was arrested.
Initial reports which have not yet been confirmed, suggest that he has now been released.
CSW Chief Executive Mervyn Thomas said, "We are deeply concerned by the detention of lawyer Nguyen Van Dai and Pastor Nguyen Trung Ton, who have both advocated for the right to freedom of religion or belief for Christians and other religious communities in Vietnam.
"We further note that these detentions took place in the same week that the EU and Vietnam held their annual human rights dialogue.
"These detentions underscore the fact that despite some improvements in the level of religious freedom and the registration of new groups in the past few years, citizens are still not free to fully exercise their human rights and to advocate for the rights of others.
"We call for the immediate release of Mr Dai and Pastor Ton, and urge the Vietnamese government to respect and protect the right to freedom of religion or belief and other human rights in Vietnam."
