A tightly-guarded Vietnamese court on Tuesday began hearing the appeal of a prominent dissident whose seven-year jail sentence sparked deep concern among Western nations.

Cu Huy Ha Vu, 53, the son of a revolutionary leader, was convicted in April of spreading propaganda against the communist state. His first trial lasted about half a day and was dismissed by Amnesty International as a "sham".

As with the earlier trial, uniformed and plainclothes police locked down the area around the court building, where an official said the appeal began on Tuesday morning.

A few of Vu's supporters gathered nearby with signs proclaiming his innocence. A witness said one person was earlier dragged away by police, who had at least three city buses on standby ready to cart people off in the event of mass arrests.

Vu's case led to an "unprecedented outpouring of popular support" across diverse sectors of society, much of it online, US-based Human Rights Watch said in a report.

The outcome of the appeal "will have important repercussions for the rule of law and freedom of expression in Vietnam", the watchdog said.

Vu, a French-trained legal expert, was accused of spreading propaganda against the state -- including a call for political pluralism -- through writings, interviews with foreign media, and Internet material since 2009.

He was charged after twice trying to sue Prime Minister Nguyen Tan Dung over a controversial bauxite mining plan and a decree that prevented class-action petitions.

His arrest came in November as political tensions rose in the leadup to the Communist Party's secretive leadership Congress in January, which confirmed Dung in office for a second term.

After Vu's conviction in April, the United States said his case raised questions about Vietnam's commitment to reform, while the European Union said Hanoi's international reputation was at stake.

Vu's appeal comes one week after Dung was formally named to his second term in office after consolidating his power, which activists fear heralds a tougher climate for dissidents in the one-party state.

Vu is the son of Cu Huy Can, who was a celebrated poet and a member of revered founding president Ho Chi Minh's provisional cabinet from 1945.