6:30 PM PT -- Police tell us Clark and another man were stopped in a '93 Toyota Supra for failing to display a front license plate. During the stop, cops say they observed in plain sight the muzzle of a handgun sticking out of a bag in the back seat.
Police then searched the vehicle and say they located 2 loaded firearms -- a rifle and a handgun.
Clark and the other man were both arrested.
NFL superstar Frank Clark is now facing up to 3 YEARS behind bars ... after prosecutors hit him with a felony charge of possession of an assault weapon Friday, TMZ Sports has learned.
We're told Clark was arrested by California Highway Patrol on March 13th after allegedly being caught with an illegal firearm shortly after midnight.
It's not the first time he's run into this kind of trouble -- as we previously reported, Clark was arrested on June 20 ... after cops say they saw an uzi in his Lambo SUV during a routine traffic stop.
Officers say the firearm was sitting inside of an open duffle bag in the ride ... so they arrested the 28-year-old Chiefs pass rusher.
According to a source close to Clark, the football player claimed the gun was NOT his ... and instead belonged to a member of his security team.
Prosecutors have NOT yet charged Clark for the June incident.
If convicted on the charge from the March incident, Clark faces up to three years behind bars.
The two-time Pro Bowler is due in court next week for a hearing on the matter.
Clark is one of the Chiefs' best players ... he's logged 14 sacks since joining K.C in 2019, and is in the middle of a 5-year, $104 MILLION contract.