Albert Barnes Commentary Acts 21:31

"And as they were seeking to kill him, tidings came up to the chief captain of the band, that all Jerusalem was in confusion." — Acts 21:31 (ASV)

And as they were about to kill him. Greek: They seeking to kill him. This was evidently done in a popular tumult, as had been done in the case of Stephen (Acts 7). They could not pretend that they had a right to do it by law.

Tidings came. The news, or rumour, came; he was told of it.

The chief captain of the band. This band or body of Roman soldiers was stationed in the tower Antonia, on the north of the temple. This tower was built by John Hyrcanus, high priest of the Jews, and was by him called Baris. It was beautified and strengthened by Herod the Great and was called Antonia, in honour of his friend Mark Antony.

Josephus describes this castle as consisting of four towers, one of which overlooked the temple, and which he says was seventy cubits high (Jewish Wars, Book 5, Chapter 5, Section 8). In this tower, a guard of Roman soldiers was stationed to secure the temple and to maintain the peace. The commander of this cohort is here called "the chief captain." Reference is made to this guard several times in the New Testament (Matthew 27:65–66; John 18:12; Acts 5:26).

The word translated 'chief captain'—ciliarcw—denotes, properly, one who commanded a thousand men. The band—speirhv—was the tenth part of a legion, and consisted sometimes of four hundred and twenty-five soldiers, at others of five hundred, and at others of six hundred, according to the size of the legion. The name of this captain was Claudius Lysias (Acts 23:26).

In an uproar. That the whole city was in commotion.