The incident happened during Friday prayers at the King Fahd mosque in Banjul, the military force sent by the Economic Community of West African States (ECOWAS) said in a statement.
During a routine search by Senegalese police at the mosque "a 9mm calibre 'Helwan' automatic pistol was discovered" with a loaded magazine, the statement said.
The arrested man, Sergeant Baboucarr Njie, admitted he was a former member of the presidential guard of ex-leader Jammeh, the regional force added.
Barrow defeated longstanding hardline ruler Jammeh at the polls in December.
He then took refuge in neighbouring Senegal on January 15, fearing for his safety after Jammeh reversed his acceptance of the election result and sought for weeks to cling to power.
The new leader was welcomed home by jubilant crowds in late January after Jammeh left the country under pressure from the 15-nation regional bloc.
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