Demetrius was a silversmith at Ephesus. He made and sold silver shrines of Artemis (the goddess Diana). He became worried about his business of selling silver idols, as Paul was converting many people in the area to Christ.
Demetrius spoke out against Paul and his companions, and started a riot. The rioters grabbed Gaius and Aristarchus (Paul's traveling companions), and tried to get to Paul, But the town clerk quelled the riot saying "if Demetrius and his fellow craftsmen have a complaint against anyone, courts are in session, and there are proconsuls. Let them bring charges against one another..." Finally the riot subsided, and the assembly was dismissed. (Acts 19:24-40).