Troopers rolled in around 5:30 p.m. to reports of brawls, blaring music, and open alcohol use across Carson Beach and neighboring Castle Island. By the time the scene was cleared at about 9:30 p.m., the Carson Beach parking lots and a portion of Day Boulevard had been shut down for hours, State Police said. No arrests were made; no injuries reported.

State Police did not provide a count of the crowd or of those ordered to leave, but troopers told Boston.com they monitored the area through the evening before reopening Day Boulevard.

Carson Beach — the popular DCR-run shoreline that hugs Day Boulevard from the Curley Community Center down to Castle Island — has historically drawn enormous crowds on warm-weather days, and Tuesday's near-90-degree spring afternoon was no exception. The beach is state property, which is why Mass State Police, not BPD, run point on disturbances there.

What began as a typical hot-day beach scene escalated into the kind of teen-mob disturbance South Boston residents have complained about for years — louder music, more alcohol, and crowds large enough that fights broke out at multiple points along the sand.

Tuesday's Carson Beach blowout falls under State Police jurisdiction, not BPD's — but the bigger neighborhood-takes-itself-over picture remains the same.