dà xíng / huó dòng / kāi shǐ qián / bǎo ān bù duì / yǐ jīng / zài / huì chǎng / zhōu wéi / shè zhì le / jǐng jiè xiàn

大型活动开始前，保安部队已经在会场周围设置了警戒线。

Before the large event began, the security forces had already set up a cordon around the venue.