A prominent Vietnamese dissident, already serving a 10-year term, has been handed an additional 11-year sentence for criticizing the ruling Communist Party while in prison, rights groups said Tuesday, underscoring Hanoi’s strict suppression of dissent.

Trinh Ba Phuong was convicted Friday by the People’s Court of Da Nang after authorities found a note in his cell reading, “Down with the Communist Party of Vietnam for violating human rights, down with the Communist court for wrongfully convicting me,” according to The 88 Project, an organization documenting human rights abuses in Vietnam. Phuong has denied writing the statement.

Vietnamese state media did not report the case, and the country’s Ministry of Foreign Affairs did not respond to requests for comment.

Elaine Pearson, Asia director for Human Rights Watch, said the conviction demonstrates Hanoi’s “zero tolerance for dissent” and came at a time when a senior European Union trade official was visiting Vietnam to strengthen ties. She urged governments to raise concerns about the crackdown on free expression and press for the release of activists like Phuong.

Under Article 117 of Vietnam’s criminal code, Phuong was accused of consistently displaying “a defiant and resentful attitude toward the communist regime.” The 40-year-old activist is known for campaigning on human rights, land rights, and environmental issues. His parents were imprisoned in 2014 for participating in protests.

Phuong gained international attention after documenting abuses during a January 2020 police raid that left an elderly farmer and three officers dead. He was arrested later that year and convicted in 2021 for spreading anti-state propaganda.

According to The 88 Project, this is the first instance of a Vietnamese political prisoner being prosecuted for statements made while already imprisoned. The group said, under international law, Phuong’s actions amounted to peaceful protest against human rights violations and his alleged wrongful conviction.