B737 First Officer reported encountering severe turbulence and taking action to avoid an over speed condition. The flight crew received clearance for a lower altitude and continued the flight with no aircraft damage or passenger injuries.

Narrative

While cruising at FL 350 and flying direct to ZZZ1; approximately 1:10 out of ZZZ; we encountered severe turbulence. We had a smooth ride until that point. The severe turbulence occurred without warning; almost like hitting a wall. The encounter caused the autopilot; and we also believe the auto throttles; to come offline. Both my Captain and I took the controls and I pulled the power back; preventing the aircraft from over speeding. It momentarily went smooth then the turbulence hit again and was more sustained. My Captain called for control of the aircraft; which I relinquished. In addition to encountering rolling action; our altitude increased approximately 700 to 800 ft. above our assigned altitude. I informed ATC that we encountered turbulence and needed a lower altitude immediately. I informed ATC we had been unable to maintain altitude. ATC assigned us FL 290; which we descended to. There were no indications of any aircraft damage. I contacted the Flight Attendants. They said all Passengers were seated during the event and everyone in the back was ok and without injury. After the encounter; we did not experience anything worse than light chop.