Lawyer Nguyen Van Dai (centre) stands during his trial in Hanoi on April 5. He was jailed for 15 years as the communist country tightens its grip on critics. (Photo by Vietnam News Agency/AFP)

A court in Vietnam has given harsh jail sentences to six prominent pro-democracy activists on charges of attempting to overthrow the communist government.

The People's Court of Hanoi on April 5 sentenced Christian lawyer Nguyen Van Dai to 15 years in prison, while Pastor Nguyen Trung Ton and journalist Truong Minh Duc were each jailed for 12 years and activist Nguyen Bac Truyen was sentenced to 11 years.

Le Thu Ha and Pham Van Troi were sentenced to nine and seven years respectively.

The defendants also face from one to five years under house arrest after finishing their sentences

Five of the defendants are members of Vietnam-based Brotherhood for Democracy.

Dai and Ha, his assistant, had been detained for 28 months before the trial. The rest were arrested last July.

"These sentences are too unfair and go against human thoughts. They shame the judicial system in Vietnam," Brotherhood for Democracy said in a statement. "We will never accept the injustice of the trial."

The court building was tightly guarded by police who brutally beat and detained supporters of the defendants.

Relatives said Truyen told judges that if he was imprisoned, many others would continue his work as they do not fear prison.

Father Anthony Le Ngoc Thanh, a rights advocate based in Ho Chi Minh City, said the government had failed by using prison to threaten the democracy movement. "Democracy is the real need of the people. Fear has disappeared among people," he said.

Father Thanh said the defendants did not bow and confess because they knew they had done the right things for themselves and the common good.