A321 Flight Attendant reported a gate agent opened an armed door at the gate in order to gain immediate access to the cabin. The escape slide did not deploy.

Narrative

I was working as FA A on flight ABCD from ZZZ to ZZZZ. We diverted back to ZZZ midway to ZZZZ as we had a passenger disturbance. Upon arriving at the gate; I sat in my jump seat with my seat belt on waiting for the pilots to give us the procedural chimes that we could stand up and disarm our doors. I kept waiting and heard nothing but when I looked out the window from my seat I could see the jet bridge right there. I then called the pilots asking if it's safe to disarm our doors and as he said yes and I took my seatbelt off while still on the phone with the pilot; the gate agent knocked on the window so I gave her the X symbol; as we are trained; as all doors were still armed and she opened the door anyways. When the door opened she and 2 managers; and the authorities were trying to storm onto the plane; I asked them all to get off as I still had to disarm all the doors and after that was done I told her she opened it while still armed and she said well I couldn't see anything so I just opened it anyways". I made the managers aware of the situation as well as the pilots who also stated that they forgot to give us the chimes to disarm the doors due to wanting the pax to remain seated for the authorities to come on board.Flight attendants were not made aware the pilots weren't going to give us the chimes notifying us it's okay to disarm our doors after getting to the gate and passenger disturbance causing diversion and deviation from SOPs.Suggestions: Better training for gate agents on opening aircraft doors so they don't get themselves and others hurt."