They are subjected to interrogation and beatings at a local police station.
Two opposition organization members are released after 14 years in prison.
Dinh Dang Dinh's family is concerned he is being denied access to proper treatment while dying of cancer.
A court says they stole state funds for personal gain.
They risk imprisonment and harassment for making the trip, the two say.
Members of the group say they were targeted for promoting human rights.
They demand a multiparty system to balance political power.