SÀI GÒN — Việt Nam convicted four democracy activists of trying to overthrow the communist government on Wednesday and sentenced them to up to 16 years in prison for promoting multiparty democracy.

The most well known of the four defendants, U.S.-trained human rights attorney Lê Công Định, received a five-year sentence after judges at the Sài Gòn People’s Court deliberated for just a half-hour. The court apparently showed leniency because Định acknowledged breaking the law during his testimony.

''From the bottom of my heart, I myself and these three other defendants had no intention to overthrow the government,'' Định told the court.

The stiffest sentence in the one-day trial was given to Trần Huỳnh Duy Thức, an Internet entrepreneur who testified that he had sought ways to improve Việt Nam’s political culture and root out corruption but insisted he had done nothing wrong.

Định, Thức and a third defendant could have been sentenced to death.

Western diplomats and human rights groups slammed the verdict, saying the defendants had been punished for peacefully expressing their political views.

The trial came as factions jockey for power in advance of next year’s Communist Party congress, and some observers have speculated that the current crackdown on dissent is connected to the upcoming political transition. Việt Nam has convicted 10 other democracy activists in the last three months.

None of them is better known than Định. In addition to handling high-profile human rights cases, he once represented Vietnamese fish farmers fighting an unfair trade complaint brought by U.S. catfish growers. During closing arguments at a 2007 human rights trial in Hà Nội, Định made a highly unusual public plea for freedom of expression.

A panel of judges found that the defendants had committed ''an extremely serious'' national security crime by joining the outlawed Democratic Party of Việt Nam and collaborating with overseas Vietnamese groups dedicated to ousting the communists. But they said some of the defendants, including Định, had shown remorse and had good personal records.

The court said it also took into account the fact that the defendants had been ''coerced by hostile forces'' from overseas who oppose Việt Nam’s communist government. Định testified Wednesday that he had been influenced by Western ideas while studying in the United States.

They were found guilty of violating Article 79 of Việt Nam’s criminal code, which prohibits ''carrying out activities aimed at overthrowing the people’s administration.''

Defendant Nguyễn Tiến Trung, who formed a student group called Việt Youth for Democracy in 2006 while studying information technology in France, received a seven-year sentence.

The last defendant, Lê Thăng Long, who was tried as an accomplice, received a five-year sentence.

All of them received from three to five years of probation.

Định said he had broken the letter of the law but had never intended to overthrow the government.

''The Democratic Party of Việt Nam called for a multiparty system and political pluralism, and my participation in this group constitutes a violation of Article 79,''Định said.

Định, the former vice chairman of the Sài Gòn Bar Association, studied law at Tulane University in Louisiana on a Fulbright scholarship.

''During my studies overseas, I was influenced by Western attitudes toward democracy, freedom and human rights,''Định testified.

Trung showed remorse during his testimony, saying he regretted joining the Democratic Party of Việt Nam and founding his student democracy group.

''My actions violated Vietnamese law,'' said Trung, 26. ''I was immature and made a mistake.''

The other two defendants denied wrongdoing and said they had only signed confessions under duress.

Thức, 43, acknowledged organizing something called the Chan Study Group, which prosecutors say was committed to undermining the government. But Thức said the group was simply formed to do research and make policy suggestions to Vietnamese leaders.

Lê Thăng Long, 42, acknowledged joining the study group, but said it was lawful.

''I’m innocent,'' Long said, testifying that he had been subjected to ''psychological terrorism'' by the security police.

Foreign reporters and diplomats watched the trial in a separate room at the court on a closed-circuit television that was sometimes inaudible. They were prohibited from bringing cameras or recording devices.

Ken Fairfax, the U.S. consul general in Sài Gòn, expressed disappointment with the verdict.

''We would like to reiterate our deep concern over the arrest and conviction of persons for the peaceful expression of their beliefs, political and otherwise, by the government of Việt Nam'' he said.

New York-based Human Rights Watch condemned the proceedings.

''Việt Nam’s hostility toward freedom of expression and peaceful dissent is becoming increasingly flagrant in the run-up to next year’s party congress,'' said Brad Adams, the organization’s Asia director.