Charles Spurgeon Commentary Acts 16:20-21

"and when they had brought them unto the magistrates, they said, These men, being Jews, do exceedingly trouble our city, and set forth customs which it is not lawful for us to receive, or to observe, being Romans." — Acts 16:20-21 (ASV)

Saying, These men, being Jews, do exceedingly trouble our city, and teach customs, which are not lawful for us to receive, neither to observe, being Romans.

They did not know that Paul himself was a Roman citizen, or they surely would not have brought such a charge as that against him.