Jake Paul is firing back at Canelo Alvarez for kicking 2 of the YouTube star's fans out of the ring during his post-fight interview on Saturday ... calling the champion an "old hater" who's ducking real competition.
Of course, Canelo and Jake have had a beef brewing for months ... with the 30-year-old boxer saying the Paul brothers are disrespecting the sport of boxing.
The feud reached new levels when fans -- donning Jake and Ben Askren's faces on their shirts -- invaded the ring after Alvarez's dominant win over Avni Yildirim .