If you want within a single day and within very confined area like a square kilometer, then the final suppression of the Nika riots on or about 18 January 532 AD is a good contender.  (See [here](https://www.thoughtco.com/the-nika-revolt-1788557) and [here](https://en.wikipedia.org/wiki/Nika_riots).) With 30,000-35,000 rioters in the Hippodrome in Constantinople, the imperial troops blocked the exits and slaughtered everyone.  

I realize there are some battles with higher one-day death tolls, but most battles take place over considerably larger areas.