WASHINGTON (Reuters) - A group of U.S. senators urged Vietnam's president on Wednesday to free a Catholic priest as human rights groups said his imprisonment justified putting Hanoi on a U.S. religious freedom blacklist.

Father Thadeus Nguyen Van Ly, a Catholic priest who had spent 16 years in prison for rights advocacy activities, was jailed for eight years in March 2007 on charges that he spread propaganda against Vietnam's communist government.

The group of 37 senators, led by Democrat Barbara Boxer and Republican Sam Brownback, urged President Nguyen Minh Triet to free the 63-year-old cleric, calling his trial "seriously flawed."

"We request that you facilitate Father Ly's immediate and unconditional release from prison, and allow him to return to his home and work without restrictions on his right to freedom of expression, association and movement," their letter read.

"Father Ly's long-standing nonviolent activities to promote religious freedom and democracy in Vietnam are well known in the United States," wrote the senators, who also included Democrat Edward Kennedy and Republican Orrin Hatch.

The Vietnamese Embassy in Washington did not immediately confirm receipt of the letter or issue comment.

During Ly's four-hour trial, he was denied access to a lawyer and was silenced by security guards when he attempted to speak, said the U.S. human rights group Freedom Now.

Maran Turner, the executive director of Freedom Now, said Ly's case and similar ones involving other religious figures justify putting Vietnam back on a U.S. list of "Countries of Particular Concern" for violations of religious freedom.

The United States, which put Vietnam on that list in 2004, lifted the designation before former President George W. Bush's visit to Hanoi in November 2006.

Redesignation "would quite possibly achieve the release of Father Ly as well as the release of other prisoners," said Turner.

Michael Cromatie, vice chairman of the U.S. Commission on International Religious Freedom, was permitted to visit Ly in prison in May. Ly was "in solitary confinement for reasons that are not clear," said Cromatie.

The commission also has called on Secretary of State Hillary Clinton to reinstate the CPC designation -- a decision often made in mid-September.

"We urge them to remember these prisoners and reinstate the CPC," he said, referring to Ly as well as other Catholic clergy, Protestants, Buddhists and other religious figures jailed for their activities.