Christian Solidarity Worldwide (CSW) is calling for the immediate release of the former president of the Kachin Baptist Convention in Myanmar.
The organisation said it was "deeply troubled" by the arrest of Rev Dr Hkalam Samson at Mandalay International Airport on Monday.
Dr Samson, Chairman of the Kachin National Consultative Assembly, was arrested as he tried to travel to Bangkok in Thailand.
CSW's Senior Analyst for East Asia Benedict Rogers said: "CSW is deeply troubled by the arrest of such a prominent and internationally respected religious leader in Myanmar, and are very concerned for his wellbeing."
Dr Samson was described by CSW as "one of Myanmar's most prominent and outspoken Christian religious leaders", and a "courageous defender of human rights".
He served as president of the KBC from 2018 until this year. Prior to that, he served for eight years as its general secretary.
In 2018, he met senior British parliamentarians and officials during a visit to London, and the following year met the US president at the White House.
Mr Rogers added, "We call on the Myanmar authorities to release him immediately, and we urge the international community to demand his unconditional release and ensure his wellbeing."