Nightwatch was a paramilitary organization of the Babylon 5 universe. Nightwatch was founded in 2259 by President Morgan Clark. The agency was under the control of the Ministry of Peace.
Nightwatch was comprised of civilians, law enforcement, and military personnel that had been recruited by Clark's regime to report on and deal with those activities seen to be a threat to Clark's regime. These activities included criticism of Clark, becoming too friendly with aliens, and anything else Clark saw as a threat. The agency was eventually empowered to arrest people without warrants, detain subversives, and spy on people without regard to people's rights.
A branch of the Nightwatch was set up on Babylon 5. Zack Allan joined the organization because he thought it was easy money. He became uncomfortable with Nightwatch activities and their disregard for civil liberties.
When armed resistance to Clark broke out in 2260 Allan would help neutralize the Nightwatch on Babylon 5. He told his Nightwatch superiors that a force of Narns was being brought in by ship to take over, and gave them details on the ship and arrival time. When the time came almost the entire Nightwatch force gathered at the dock, whereupon Allan sealed them in.
Captain Sheridan then arrived and placed the entire force under arrest for attempted mutiny and failure to obey the military chain of command. The Nightwatch members were forced to come out of the dock one person at a time, and surrender their equipment as they came out. He then revealed that the local Narn population was going to help with security, hiding a lie within the truth.
After Babylon 5 left the alliance and in the aftermath of the battle that followed all known Nightwatch members were gathered up and transported back to Earth. A small covert group remained to use terrorism to bring Sheridan and his officers down. This group was all captured or killed, with Sheridan himself nearly killing the Nightwatch member who severely injured Ambassador Delenn.
While the Babylon 5 branch of Nightwatch was put out of business, Nightwatch as a whole remained active through the rest of the Clark regime. After the liberation of Earth the Nightwatch was finally disbanded.