The United States and international rights groups have also urged authorities in Hanoi to free Nguyen Van Ly, who is in his 60s and was sentenced to eight years in prison in 2007 for propaganda against the state.

Prosecutors in Vietnam say Ly - who was taken into custody on Monday - was a founding member of the banned "Bloc 8406," considered by analysts as the first organized pro-democracy coalition inside the country.

"We are disappointed that Father Nguyen Van Ly has been returned to prison, and we remain very concerned about his health," Canadian Foreign Minister John Baird said in a statement.

"Canada urges the government of Vietnam to permanently release Father Ly and the many others imprisoned for peacefully expressing their opinion," Baird said.

"Our government will continue to pursue Canada?s principled approach to foreign policy to advance freedom, democracy, human rights and the rule of law."

Ly's arrest followed months of uncertainty over his sentence after the expiry of a one-year suspension of his prison term ordered by a court in March 2010.

In March this year, a government spokeswoman said Ly would be sent back to jail if his health returned to normal, but there was no official follow-up until he was taken into custody on Monday.

Agence France Presse - July 28, 2011