At least eight political activists have been arrested on subversion charges in Vietnam in a crackdown which began after the prime minister was re-appointed, their legal adviser said on Tuesday. The suspects, who have been involved in recent anti-China protests and other activities, were rounded up in Ho Chi Minh City, Hanoi and the north-central province of Nghe An, Le Quoc Quan told AFP. The eight, all of them belonging to the minority Catholic faith, have been formally arrested for "activities aimed at overthrowing the people's administration", he said. They include Paulus Le Van Son, a blogger. Rights campaigners have expressed concern that the re-appointment of Prime Minister Nguyen Tan Dung for a second term in late July heralds a tougher climate for dissidents in the authoritarian one-party communist state. The suspects are among at least 13 activists targeted in the crackdown who are still being held, Quan said, adding there were "many reasons" why the other five may have been detained. There was no immediate comment from Vietnamese officials. Vietnam says it has achieved significant progress on human rights. Under the country's legal system, people can be held for initial questioning before being formally arrested on a charge. Quan said he was familiar with the suspects' cases through his membership of the Vinh Diocese Committee for Peace and Justice, and his role as chief of the management board of a Catholic professionals' association. "I am worried for them," he added. He said the activists had participated in recent demonstrations against China's actions in the South China Sea, where maritime tensions between Hanoi and Beijing escalated this year. They had also studied "non-violent struggles", signed a petition for the release of prominent dissident Cu Huy Ha Vu, and opposed a controversial bauxite mining project in the Central Highlands, Quan said. The anti-subversion charge has been used against other dissidents, including French-Vietnamese lecturer and blogger Pham Minh Hoang who was sentenced to three years in prison this month. His sentencing was the latest Vietnamese judicial decision to raise concerns from Western governments, after a seven-year jail term was upheld in early August for the dissident Vu; and the re-incarceration in July of a Catholic priest, Nguyen Van Ly, who has a brain tumour. On Monday officials said two democracy activists were among more than 10,000 prisoners granted amnesty to mark September 2 National Day.