"We are merely sentries, Captain Picard. We watch and protect our people." - unknown female Iconian to Jean-Luc Picard. (TNG - Gateways short story: "The Other Side")

The Sentries were an organization that existed within the Iconian civilization.

This agency was formed by the remnants of the Iconian Empire after they had fled the devastation of their home world when it was orbitally bombarded by races that feared them due to their gateway technology. After departing known space, the Iconians formed a body of their people who gathered when their equipment was in use. Under their laws, they were required to monitor the use, record the species that employed the gateways and watch them for signs of incursion. These rules that governed them told them to simply watch and remain vigilant in case they were threatened which they obeyed. (TNG short story: "The Other Side")

In 2376, during the Gateways Crisis, the Sentries were aware of the activation of the first gateway by the Petraw though it had been so long that such an alert had been initiated that the leader of the group had to consult the computer records in order to find out what the signified and what would be their next course of action. (TNG - Gateways short story: "The Other Side") Kira Nerys met a custodian of the gateways who aided her in returning her to her own timeline after she had been accidental sent into Bajors past. (DS9 - Gateways short story: "Horn and Ivory") (Starfleet Captain Jean-Luc Picard met a gathering of Sentries who informed him of their mission. (TNG - Gateways short story: "The Other Side")