Cast off their clothes - Their outer garments. Probably they did it now intending to stone him, Acts 7:58.
And threw dust into the air - As expressive of them abhorrence and indignation. This was a striking exhibition of rage and malice. Paul was guarded by Roman soldiers so that they could not injure him; and their only way of expressing their wrath was by menaces and threats, and by these tokens of furious indignation. Thus, Shimei expressed his indignation against David by cursing him, throwing stones at him, and casting dust, 2-Samuel 16:13.

Cast off their clothes - Bishop Pearce supposes that shaking their upper garments is all that is meant here; and that it was an ancient custom for men to do so when highly pleased or greatly irritated; but it is likely that some of them were now actually throwing off their clothes, in order to prepare to stone Paul.
Threw dust into the air - In sign of contempt, and by way of execration. Shimei acted so, in order to express his contempt of David, 2-Samuel 16:13, where it is said, he cursed him as he went, and threw stones at him; or, as the margin, he dusted him with dust. Their throwing dust in the air was also expressive of extraordinary rage and vindictive malice. The apostle, being guarded by the Roman soldiers, was out of the power of the mob; and their throwing dust in the air not only showed their rage, but also their vexation that they could not get the apostle into their power. It is still used as a token of hostility and defiance. M. Denon, (Travels in Egypt, vol. iii. p. 98), on coming down the Nile to Cairo, stopped at the ancient city of Antinoe, to examine its ruins. "Being desirous of obtaining a view of the whole of these ruins, we ascended a little hill, and soon perceived the inhabitants of the modern village assembling behind an opposite eminence: scarcely had we come over against them than, supposing our intentions to be hostile, they called out for assistance, and threw dust into the air, in token of defiance. The alarm spread, and they began firing upon us."

And as they (c) cried out, and cast off [their] clothes, and threw dust into the air,
(c) The description of a seditious tumult, and of a foolish and mad multitude.

And as they cried out,.... In this furious manner:
and cast off their clothes; either like madmen, that knew not what they did, or in order to stone him; see Acts 7:57.
and threw dust into the air either with their hands, or by striking the earth, and scraping it with their feet, through indignation and wrath, like persons possessed, or mad.

They rent their garments - In token of indignation and horror at this pretended blasphemy, and cast dust into the air - Through vehemence of rage, which they knew not how to vent.