While they were trying to kill him, news reached the commander of the Roman troops that the whole city of Jerusalem was in an uproar.
He at once took some officers and soldiers and ran down to the crowd. When the rioters saw the commander and his soldiers, they stopped beating Paul.
References for Acts 21:32
The commander came up and arrested him and ordered him to be bound with two chains. Then he asked who he was and what he had done.
References for Acts 21:33