Details are very light; however, an armed man wearing body armor was arrested shortly before President Trump arrived at the American Airlines arena in Dallas Texas.
According to NBC5 Dallas:
Dallas police arrested the man while he was wearing a helmet, a vest and a backpack. Officers confiscated a gun, a breathing mask and an aerosol can. The man, who has not been identified, was arrested near counter-protestors. (link)
According to additional reporting the gunman was located with of a group of protesters against the U.S. withdrawal from Syria.