The protesters took to the streets with their faces covered.
Tips
usage
Often used as an attributive: 蒙面人 (masked person), 蒙面歌手 (masked singer), 蒙面劫匪 (masked robber). The CC-CEDICT also notes an older sense of 'brazen/shameless' (literally having a thick face covering), though this is rare in modern usage.
