。
The riot was quickly suppressed.
Literal
Riot [topic-は] immediately was-suppressed.
鎮められた is the passive of 鎮める ('to suppress, calm, quell') — a more literary verb than the everyday 抑える. すぐに is the casual 'immediately'; the more formal 直ちに carries an extra weight common in news writing or official statements. The agent doing the suppressing is omitted entirely, which is typical of passives in Japanese reporting style: focus stays on the event, not the actor.