Any police officer or law enforcement officer and stop and you in the United States (depends on jurisdiction).
Officers can make arrests, even when not in uniform. They retain their powers on and off duty. If they are “off duty,” they may face certain restrictions (legally or administratively)); such as being prohibited from wearing their uniform while employed at a second job offering security.
When they are off duty, they also have the same power as any citizen to make a “citizen’s arrest” if they witness a felony.