Official LGBT+ supporters' groups of both clubs -- Pride of Irons and Chelsea Pride -- said on Twitter that they had witnessed continuous discriminatory chanting directed towards Chelsea fans at Stamford Bridge and reported it to West Ham.
The BBC said on Monday that the Metropolitan Police were contacted on social media and are conducting enquiries. No arrests have been made yet.
West Ham issued a statement on Monday saying they do not condone any discriminatory behaviour by supporters.
"West Ham United in no way condones any behaviour that falls short of the highest standards which the club sets when it comes to equality," a club spokesperson told Sky Sports.
"At London Stadium against Arsenal on Monday we will be celebrating and demonstrating the great work the club does to create a welcoming and inclusive environment for ALL supporters, including our LGBT+ fanbase, as part of our ongoing and unwavering support for the Rainbow Laces campaign."
Football equality body Kick It Out tweeted https://twitter.com/kickitout/status/1201448892807680001 that it had told the FA the chants were homophobic and had to be treated as such.
LGBT equality charity Stonewall said in a tweet https://twitter.com/stonewalluk/status/1201093080167923713 that tackling anti-LGBT slurs was vital to make more sport inclusive.
Professional players across Britain will help promote equality and diversity during the next two rounds of games as part of the Rainbow Laces campaign https://www.premierleague.com/RainbowLaces, including using rainbow-coloured captain's armbands and laces. (Reporting by Shrivathsa Sridhar in Bengaluru; Editing by Ken Ferris)