HANOI - Communist Vietnam has defended its commitment to freedom of speech in the face of widening global opposition to the recent arrest of a prominent lawyer and writer.

Le Cong Dinh, 40, of southern Ho Chi Minh City, was arrested at the weekend for allegedly colluding "with domestic and foreign reactionaries to sabotage the Vietnamese state", the government's Vietnam News Agency said.

Vietnam has a "consistent policy to guarantee citizens' freedom of speech and opinions as well as encourage all people to give recommendations", foreign ministry spokesman Le Dung said on the government's website.

But global human rights watchdog Amnesty International, in a public appeal for his release issued Thursday, called Dinh "a prisoner of conscience, detained solely for the peaceful exercise of his right to freedom of expression".

Dinh had written commentaries on legal issues in prominent Vietnamese newspapers and was part of the legal team that appealed the conviction of two other lawyers, Nguyen Van Dai and Le Thi Cong Nhan, in 2007.

The appeal court upheld their convictions on charges of spreading propaganda against the state but reduced their prison terms by one year each, to four years for Dai and three for Nhan.

Like the two lawyers he defended, Dinh has been charged under the Penal Code's Article 88, which bans "propaganda" against the state.

The Bangkok-based Southeast Asian Press Alliance (SEAPA), a coalition of journalistic and press freedom groups, called for Dinh's release. It said the arrest sent a "chilling message" to lawyers, writers, and advocates of peaceful change.

US-based Human Rights Watch and the press freedom watchdog Reporters Without Borders (RSF), from Paris, have also criticised the arrest, which the United States said "contradicts the government's own commitment to internationally-accepted standards of human rights and to the rule of law".

Vietnam's foreign ministry spokesman responded that the arrest "was conducted in line with Vietnamese laws".