Air Carrier Captain reported a critical ground conflict while taxiing into the assigned gate. A belt loader driven by another airline employee passed very close under the aircraft's nose requiring the Captain to aggressively apply brakes.

Narrative

After waiting (#2 engine shutdown) on the taxi line short of gate for complete ramp personnel to assemble for final approx 30 yds of taxi required to reach the gate stop point; I then was directed to taxi forward by the Marshaller. I began my taxi- aircraft moving. 4 seconds later; I then heard my first officer plus my jumpseater holler 'STOP!' Then; the Marshaller gave a Stop (wands crossed) signal quickly thereafter. I immediately stopped the jet ; set the brakes to assess. I saw a belt loader emerge from under the cockpit to my left and traveling to my left at high rate of speed (25-30mph). He appeared to be another airline employee driving the belt loader. He continued at a high rate of speed under the bridge to the south and disappeared. We then continued taxi to gate stop point without incident. I estimate the belt loader came within 10 feet of the nose gear at high rate of speed while the aircraft was moving- terribly egregiously countering safety and experiencing a near-miss during ground operations. ZZZ gate agent was an eyewitness; as was all ramp personnel (wing walkers) tasked to guide us in. The Marshaler refused to meet me for debriefing afterward. But the ZZZ came to the flight deck instead the of Marshaler. He knew nothing of the incident; but wanted information from me regarding this incident. With complete disregard for ground operation safety; the driver of the belt loader was traveling (1) In an area of the ramp where he shouldn't have been; (2) At a high rate of speed; (3) Driving underneath a moving jet.