picket line

a line of strikers or other demonstrators serving as pickets.

Origin of picket line

First recorded in 1855–60

A brief conversation took place between the general and Captain Brown of the picket-line.

Who Goes There? |Blackwood Ketcham Benson
Carleton often rode through them, and out on the picket-line.

Charles Carleton Coffin |William Elliot Griffis, D. D.
Would it not be well to have an intermediate man between me and our picket-line?

Who Goes There? |Blackwood Ketcham Benson
One day the guard brought in a suspicious character from the picket-line.

Whip and Spur |George E. Waring

And the tired men snuggled down under their blankets, not very well pleased with the picket-line.

Company G |A. R. (Albert Rowe) Barlow

picket line

a line of people acting as pickets