The participants told journalists they were protesting over the physical abuse of a group of people, including four monks, jailed for allegedly stealing religious artifacts.
They say one of the monks, a 70-year-old, died as a result of the beating.
The monks are said to have been members of the state-sponsored Buddhist Church of Vietnam.
Late last month, the court in nearby Bac Giang province ordered the temporary release of the seven people still in custody, citing a lack of evidence against them and ordering police to investigate further.