stand guard
stand watch
keep guard ,
stand sentinel

Some police came into the room and started talking to the ones who had been standing guard.

Would you stand guard here for the next hour or so?

He'd be here soon, to stand guard over her for the night.

I stand guard so that all the rest of you can play.

As always, one among them stood guard for the rest.

Then we were started across with both of our leaders standing guard.

She could see him standing guard, his back to her.

Three guys are standing guard in front of the building.

The young men standing guard in front of our building kept watch through the night.

"You people do have training on how to stand guard?"