LOWELL — Chaos erupted on the very first day of classes at UMass Lowell as the university went into lockdown over reports of a possible gunman roaming South Campus.

Sirens wailed and alerts blasted phones just after noon, ordering students to shelter in place as heavily armed officers swarmed Riverview Suites.

Police said they were hunting for a man suspected of carrying a weapon — but so far have not said whether anyone was hurt or whether shots were fired.

The chilling scare came as thousands of students returned to campus for the fall semester, leaving classrooms dark and cafeterias abandoned while cops in tactical gear searched for the suspect.

Rep. Lori Trahan, who represents Lowell in Congress, rushed to social media to assure constituents she was “actively monitoring” the situation and “in touch with local officials.” She urged anyone in the area to follow police instructions.

By mid-afternoon, cruisers, K9 units, and officers were clustered around the dorms as students peeked from windows and families anxiously texted loved ones.

It’s still unclear who the man is, what weapon he was carrying — or whether the lockdown threat will stretch into the night.