Chelsea has asked police to investigate racist abuse directed at midfielder John Obi Mikel, while black Tottenham players were apparently taunted during a Europa League match.
Racist tweets were sent to Mikel after the Nigeria international's mistake in Wednesday's Champions League opener against Juventus led to Chelsea conceding an equalizer.
Chelsea says "we've informed the police and support taking the strongest possible action."
Mikel deleted his Twitter account Thursday.
On Thursday night, Lazio fans allegedly aimed monkey chants at Tottenham players Jermain Defoe, Aaron Lennon and Andros Townsend.
Tottenham manager Andre Villas-Boas didn't hear the abuse, and says "it's for the authorities (UEFA) to follow up in any kind of investigation."
On Wednesday, British lawmakers urged football authorities to do more to combat racism in football.