"Basically every criminal in the country" without regard for charging them with any particular crime. Guilt by popular reputation, guilt by association, guilt by tattoo, guilt by suspicion or dislike. Indefinite sentence. No trial. Prison conditions as proudly abusive & humiliating as diplomatically feasible.

This is martial law, an internal military occupation with dictatorial control. An act of desperation welcomed only because El Salvador was previously, for decades, a bit of a failed state undergoing something tantamount to a low-grade civil war.

Martial law in the developing world doesn't have a history of going very well for very long. It's a brittle situation.