The United Nations human rights office highlighted the cases of two prominent activists -- Tran Thi Nga and Nguyen Ngoc Nhu Quynh -- who in recent weeks were jailed for nine and 10 years respectively.
Both were punished under article 88 of Vietnam's penal code, which prohibits "anti-state propoganda".
The UN condemned the "flawed judicial proceedings" that led to both convictions.
In Nga's case, the 40-year-old best known for her anti- China activism was held incommunicado from her arrest in January until a few days before her trial, which lasted one day.
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"We urge the Vietnamese authorities to immediately release all those detained in connection with their exercise of their rights to freedom of expression, and to amend the overly broad ill-defined laws that are used -- under the pretext of national security -- to crack down on dissent," the rights office said in a statement.
It added that many activists had been "intimidated, harassed and brutally beaten".
"Human rights defenders should never be treated as criminals who are a threat to national security," the statement said.
Dozens of activists are imprisoned in the country, where all independent media is banned and public protests are routinely broken up, often violently.