listen up!

Listen up! {infml} — Тихо! Внимание!

Used for getting the attention of a noisy group of people and making them listen to what the speaker is going to say.

Example 1: Hey, listen up everybody!
Example 2: Okay, people. Listen up.

It's a little informal. A more polite way of saying this at a formal gathering would be

May I have your attention please.