Air carrier First Officer reported that while being marshalled into the safety area; a tug drove in front of the aircraft. The Captain applied immediate braking to avoid a collision.

Narrative

At XA29 on Day 0; I was acting as PM on a Flight from ZZZZ [airport] to ZZZ [airport]. We were making the left turn to park at a stand on the Southeast Ramp in ZZZ. The safety area was clear; a marshaller and wing walkers were in place; and the lead marshaller was actively marshalling us. All of a sudden; a tug violated the safety area from our left; entering the silhouette at a rapid rate of speed in front of our left wing; passing in front of our #1 engine; crossing in front of us on the lead-in line; and finally parking behind the air stairs. The lead marshaller signalled for us to stop; and the captain immediately applied braking to stop the aircraft; our nose offset approximately 45 degrees from the lead-in line at this point. The marshaller gestured at the tug driver to get out of the silhouette; to which the tug driver simply shrugged in response. As soon as the tug cleared the safety area; we resumed parking without incident. After speaking with ground personnel over the intercom (not the tug driver; but someone who claims to have spoken with them after the flight); we ascertained that the tug driver claimed to have seen the air stairs violating the safety area; and that was their reasoning for cutting us off during the parking process. However; the air stairs were not adjusted or otherwise moved after the tug cut us off; and only began to move once we had parked. I would like to emphasize that at the start of this event; we were entering the stand to park; we were actively being marshalled; and both myself and my captain had verified that the entire safety area was clear of any hazards to the aircraft; including the air stairs.Cause: The tug driver's decision to violate the safety area; whether intentionally or absent-mindedly; while we were in the process of parking the aircraft.Suggestions: Ground personnel training and overall situational awareness should both be increased to the point where these incidents never happen.