It is understood that Hyeon Soo Lim had been making regular humanitarian mission trips to the secretive state before he was detained last February and campaigners fear he has been abused and tortured.
The Korean-Canadian pastor, who is in his early 60s from Toronto, has reportedly been sentenced to hard labour for life for "crimes against the state."
His offences are said to include trying to use religion to destroy the North Korean system, helping North Koreans leave the country and harming the dignity of the supreme leadership.
It is thought that Revd Lim has made more than 100 trips to North Korea over the last two decades, working with an orphanage, schools and a nursing home, and supplying food, textbooks and agricultural tools.
Those behind a petition calling for his release say Revd Lim's trips were not political in nature and they suspect he was forced into making a confession.
They said Canada's Prime Minister Justion Trudeau and Canada's Global Affairs Department are reportedly among those expressing concern for Revd Lim.
Campaigners say North Americans held captive in North Korea have been successfully released in the past and Rev. Lim's release is "more than possible".
In a statement posted along with the petition, they said: "North Korea has very strict rules against any missionary or religious activities, as they are threatening to the supremacy of totalitarian leadership. Merely leaving a Bible in a public place can lead to arrest and severe punishment.
"How will Prime Minister Trudeau and United Nations Secretary General Ban Ki-Moon respond to a humanitarian imprisoned - and most likely abused and tortured - in the world's most atrocious regime?
"The Canadian government and the United Nations need to take strong, affirmative action NOW. If there is political will, Pastor Lim can be rescued."
The campaigners are using the Twitter hashtags #freepastorlim and #bringlimhome