A Vietnamese appeal court on Friday upheld jail sentences against three activists who were convicted for "public disorder" offences earlier this year following short trials that generated widespread criticism.

Bui Thi Minh Hang, a high-profile anti-China activist and the most prominent of the three, was imprisoned for three years in August following a one-day trial marked by strict security including the detention of scores of supporters.

Hang, 50, was convicted alongside Nguyen Thi Thuy Quynh, 28, and Nguyen Van Minh, 34 after they were arrested in February while trying to visit a former political prisoner who lives in Dong Thap province in the southern Mekong Delta region.

The trio's sentences were upheld following a half-day appeal trial in the province on Friday.

"All defendants did not admit their crimes. So the appeal court upheld their sentences," lawyer Tran Thu Nam told AFP.

Social media reports, which could not immediately be verified, said almost 20 activists were taken into custody by security forces as they tried to demonstrate outside the courthouse.

Vietnam's authoritarian rulers are sensitive to any public criticism of their socio-political policies, especially in the handling of the current territorial dispute with Beijing in the South China Sea.

Anti-China activists in Vietnam are frequently harassed, arrested and face lengthy jail terms.

The one-party state, which bans private media, is regularly denounced by rights groups and Western governments for its intolerance of political dissent.

"In political cases like this, nothing in the courts is even remotely independent of the ruling Vietnam Communist Party," Phil Robertson from Human Rights Watch said in a statement published before the trial.

"What happens has been dictated in advance, so these appeals cases are best understood as a second sentencing hearing and a judgment by the authorities on how cooperative the activists have been in prison so far," he added.

At the time of their original convictions, the US embassy in Hanoi said it was "deeply concerned" by the sentences.

According to Reporters Without Borders, Vietnam is currently holding at least 34 bloggers in prison.

Last week, police arrested well-known commentator Nguyen Quang Lap for anti-state charges. He was the second writer seized in just a fortnight as authorities keep up their long-running crackdown on online dissent.