to fly a flag at half-mast 下半旗

to fly a flag at half-mast
繁：下半旗
简：下半旗

特首林鄭月娥昨決定，政府總部今日將下半旗致哀，主要官員、行政會議成員及公務員將於早上10時站立默哀；另農曆大年初二（周六）晚的煙花匯演亦會取消。

九巴車長控危駕致死 今提堂 政總今下半旗 年初二煙花取消（2018年2月13日）。明報，取自 https://news.mingpao.com/pns/dailynews/web_tc/article/20180213/s00001/1518458456693

The flags of Hong Kong and the mainland were flown at half-mast at the government headquarters on Tuesday as officials and legislators observed a minute’s silence to mourn the 19 people killed in the city’s deadliest crash in nearly 15 years.

Flags flown at half-mast as Hong Kong mourns victims of deadliest bus crash in 15 years. (2018, February 13). South China Morning Post. Retrieved from http://www.scmp.com/news/hong-kong/community/article/2133125/hong-kong-leader-carrie-lam-skip-lunar-new-year-night