siren

In Zhuhai, police patrolled the streets with sirens and megaphones, telling people to stay at home.

Emergency sirens, lawn mowers and music in a nightclub, by contrast, can exceed 90 decibels.

Instead, she said, she began to hear the sound of sirens as she approached Hart Street and was confronted by a scene "out of a film set" with wounded children lying on the ground.

A video posted on social media shows the siren sounds blaring inside the usually tranquil British Library in London.

At 15:00 BST, compatible phones will vibrate and make a siren sound for 10 seconds, even if set to silent.