Narrative:

Standard taxi to xxr. Flight in an unusually long departure line up of approximately eight to ten aircraft ahead and the same behind our flight. Tower controller stated over tower frequency that no line up and wait clearances would be given because he was working both ground and tower frequencies. Eventually aircraft Y was given takeoff clearance ahead of us. Then we were given a takeoff clearance from xxr. I taxied onto the runway slowly to give time for aircraft Y to rotate off runway. As I lined up for takeoff both the first officer (first officer) and myself saw an aircraft crossing our runway. I brought the aircraft to a stop not having yet set takeoff power. It was then the rwls lights (runway status lights) came on (I thought a little late). My first officer told the tower we would hold and wait. The tower hesitated and said something about line up and wait. The crossing aircraft cleared our runway and we were reissued another take off clearance. Rest of flight normal.I believe this event occurred because the ATC controller was working the west side of the airport's two frequencies by himself during an unusually busy bank time late at night.I am not sure why the west side of ZZZ was so busy at that time. It did not seem normal. Even a great ATC controller cannot work this much traffic alone. Keep additional controllers around if there are unusual traffic events occurring.

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Original NASA ASRS Text

Title: A321 flight crew reported being cleared for takeoff with another aircraft crossing the runway.

Narrative: Standard taxi to XXR. Flight in an unusually long departure line up of approximately eight to ten aircraft ahead and the same behind our flight. Tower Controller stated over tower frequency that no line up and wait clearances would be given because he was working both ground and tower frequencies. Eventually Aircraft Y was given takeoff clearance ahead of us. Then we were given a takeoff clearance from XXR. I taxied onto the runway slowly to give time for Aircraft Y to rotate off runway. As I lined up for takeoff both the FO (First Officer) and myself saw an aircraft crossing our runway. I brought the aircraft to a stop not having yet set takeoff power. It was then the RWLS lights (Runway Status Lights) came on (I thought a little late). My FO told the tower we would hold and wait. The tower hesitated and said something about line up and wait. The crossing aircraft cleared our runway and we were reissued another take off clearance. Rest of flight normal.I believe this event occurred because the ATC Controller was working the west side of the airport's two frequencies by himself during an unusually busy bank time late at night.I am not sure why the west side of ZZZ was so busy at that time. It did not seem normal. Even a great ATC controller cannot work this much traffic alone. Keep additional controllers around if there are unusual traffic events occurring.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.