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Ronnie Ortiz-Magro from "Jersey Shore" is in police custody after allegedly striking Jen Harley, chasing her with a knife and, eventually, forcing cops to tase him ... TMZ has learned.

Ronnie and his GF, Jen -- who've been on and off due to several violent incidents -- got into another heated argument early Friday morning, shortly after 2:30 AM, at an Airbnb they're renting in L.A. ... according to our law enforcement sources.

As we've seen in the past ... the argument got out of hand quickly, because we're told Ronnie allegedly slapped and punched Jen ... who then went running out of the house, screaming for help.

Police got multiple 911 calls reporting the fact Jen was in distress.

Our law enforcement sources say Ronnie gave chase ... running after Jen, allegedly with a knife in his hand, and horrifyingly, also with their infant daughter! We're told Ronnie was holding 18-month-old Ariana as he yelled at Jen to get back in the house.

We're told Ronnie eventually dropped the knife and locked himself and Ariana in the house. That's where he was when police arrived on the scene, and he did not respond to their commands to come outside.

We're told police decided to break down the door, because they were worried Ariana could be in danger. According to our law enforcement sources, Ronnie still would not go peacefully ... so officers had to use a taser to take him down.

Fox 11 LA, which broke the story, got video of Ronnie on a gurney in cuffs after being subdued.

Cops took Ronnie to a hospital to get checked out ... and we're told he will be booked as soon as doctors give the okay.

Paramedics on the scene checked out Jen and the baby, and neither suffered major physical injuries. We're told Jen did have visible marks on her face.

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Mind you, this all unfolded less than 12 hours after we saw Jen and Ronnie hand-in-hand in Bev Hills, declaring they're back together.

They told us they were ignoring all the advice from friends and family, who -- as we've reported multiple times -- believe they have an absolutely toxic relationship.

Back in July, Jen was just cleared of domestic battery stemming from an alleged fight that happened in Vegas on New Year's eve, when Ronnie claims Jen threw an ashtray at his head.

Harley claims when she returned home later that night, her place was ransacked and she, of course, believes it was Ronnie. She accused him of smashing a flat-screen TV, picture frames and a vase. Ronnie went on to file a battery report Jan 3 which she was then arrested for in May.