Tanzanian authorities on Wednesday arrested former Nation Media Group journalist Muthoki Mumo and former Mail & Guardian editor Angela Quintal in unclear circumstances.
The Committee to Protect Journalists (CPJ) has demanded their release after their detention while on a reporting in Tanzania.
Ms Quintal is the CPJ's Africa programme coordinator while Ms Mumo is the sub-Saharan Africa representative.
CPJ in a statement said that officers who identified themselves as Tanzanian immigration authority officers, detained Quintal and Mumo in their hotel room in Dar es Salaam and confiscated their passports.
They two were then escorted from the hotel and taken to an unknown location.
"We are concerned for the safety of our colleagues Angela Quintal and Muthoki Mumo, who were detained while legally visiting Tanzania," Joel Simon, CPJ's executive director said.
"We call on the authorities to immediately release them and return their passports."