The suspect was spotted about 2:20 p.m. in the area of Alameda and Ord streets, just north of Union Station, according to the Los Angeles Police Department.
Officers responded to the scene and took 54-year-old Lamar Edwards into custody without incident as he walked along Cesar Chavez Avenue, an LAPD spokesperson said. He was later formally arrested on suspicion of being a felon in possession of a weapon.
The ammunition magazine in the rifle was fully loaded at the time, police said.
Authorities found no evidence that Edwards had pointed the firearm at anyone.
He was being held without bail.
CORRECTION: An earlier version of this story reported the weapon as an AK-47, but the type of weapon is believed to be a highly modified SKS, an expert told ABC7.