Air Carrier Captain reported the aircraft collided with a belt loader that was not positioned properly. The aircraft's left engine hit the loader while taxiing into the gate. The Captain was following the hand signals from the marshal.

Narrative

When I arrived at gate in ZZZ there was a marshaller and 2 wing walkers at the gate. After making the left turn from RM to the J line we noticed that there were obstacles past the solid red lines of the Operational Safety Zone and stopped the aircraft as the wing walker also gave the stop command to us. Once these were removed by a tug we were given the signal to continue taxiing in on the J line from the lead marshaller. I continued the taxi in at a very slow and cautious speed and noted that there were no more obstacles inside of the solid red lines of the Operation Safety Zone. Under marshaller guidance I continued until he raised his arms up and finished the marshalling procedure with arms raised up and slowly issuing a stop. The aircraft was stopped on the J line and on the 77-2 line. However; the marshaller never noted that there was a belt loader on the left side of the aircraft while continuing the marshalling in to the parking position. This led to the left engine making contact with the belt loader. It appeared that once we were parked someone came up to the lead marshaller to tell him that the plane hit the belt loader. At no time was I able to see that far back to the engine or where the belt loader was (outside of the red line and not inside of the operational safety zone) as I was watching the marshaller during the taxi in to the gate. After determining what happened I called the duty pilot and debriefed her about what was happening and coordinated how to get the passengers off the plane in a timely and safe manner. There were no injuries.