Reuters last had contact with its stringer early on Wednesday before he went to report on the demonstrations, which resulted in clashes between police and protesters.
An official in the external information council, contacted by Reuters, did not say whether charges would be brought against the two Sudanese journalists. The official had earlier said they would be released early on Thursday.
"We do not know the circumstances of the detention and are actively seeking additional information about the situation," a Reuters spokesperson said.
AFP confirmed the detention of one of its journalists. In an article, the news agency said its management strongly condemned the arrest and asked for his immediate release.
(Reporting by Samia Nakhoul and John Davison; Editing by Nick Tattersall and Richard Balmforth)
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)