A Vietnamese court has jailed three activists for up to three years, a lawyer said Tuesday, after a one-day trial marked by strict security including the detention of scores of supporters.

Bui Thi Minh Hang, a high-profile anti-China activist and the most prominent of the three, was given a three-year jail term by the court in the southern Mekong Delta province of Dong Thap.

She was found guilty of "causing public disorder", her lawyer Tran Thu Nam told AFP.

Hang, 50, was arrested alongside the two other activists in February when they were on their way to visit a former political prisoner who lives in Dong Thap province.

Rights activist Nguyen Thi Thuy Quynh, 28, was jailed for two years.

Nguyen Van Minh, 34, who is an independent Hoa Hao Buddhist practitioner, received a two-and a-half year jail term. All faced the same charges.

"All the defendants denied the charges against them and pleaded not guilty," lawyer Nam added.

Scores of people trying to attend the trial were detained, according to activists and online reports.

"About 60 or 70 people -- they arrested them and beat them at the police station," activist Nguyen Lan Thang told AFP by phone from Dong Thap province.

The US embassy in Hanoi said it was "deeply concerned" by the convictions.

"The use of public disorder laws by Vietnamese authorities to imprison government critics for peacefully expressing their political views is alarming," it said in a statement, calling for their immediate release.

New York-based Human Rights Watch has slammed the trial. Vietnam was "resorting to bogus traffic offences to criminally prosecute activists," Phil Robertson, HRW's deputy Asia director, said in a statement.

Hang had previously been detained at a "re-education centre" for two years in 2011. Minh and Quynh were not well-known in Vietnam.

The trial comes as a senior politburo member, Le Hong Anh, is in Beijing for high-level talks aimed at easing tensions following violent anti-China riots in Vietnam in May.

Hanoi is locked in a bitter maritime territorial conflict with Beijing over disputed waters and island chains in the South China Sea.

In May Beijing moved a deep-sea oil rig into waters that Hanoi claims, triggering waves of protests across Vietnam in which at least three Chinese citizens died and scores of foreign-invested businesses were destroyed.

Diplomats say there have seemingly been fewer high-profile convictions of activists this year as Hanoi has sought support in the territorial dispute from the international community.

Former wartime foe Washington has suggested it could lift a ban on the sale of lethal weapons to Vietnam, introduced due to human rights concerns, pointing to purported improvements in Hanoi's rights record.

A number of activists, including Cu Huy Ha Vu, one of the country's most high-profile dissidents, have recently been released. Vu now lives in the US.

Vietnam, a one-party state, is regularly denounced by rights groups and Western governments for its intolerance of political dissent and systematic violations of freedom of religion.