The Blackburn forward is said to have shouted 'F*** off, white boy' to a pitch-side helper who failed to return the ball quickly during Sunday's 3-0 defeat to Everton at Goodison Park.
Fans have reacted with venom on football forums and websites, with some going so far as to issue death threats, according to the Daily Star.
Welcome to Goodison: Everton fans show what they think of El Hadji Diouf
An Everton fan, whose screen name is Sticky Toffee, threatens to 'trackdown the scum'. He added: 'This f***ing idiot deserves to be taught alesson, he should be hunted down. No ground will be safe.'
Another warns: 'He's the most hated man on Merseyside. now the rest of Britain hate him too.'
Vitriol: Blackburn forward Diouf has been described as 'absolute vermin' by an angry fan on the internet
Another blasted: 'I hate this guy more than any other player. Absolute vermin.'
Diouf, 28, has a history of trouble with the FA, having three times been charged with spitting at opponents or fans.
He was fined £5,000 in 2003 after spitting at a Celtic fan while playing for Liverpool, £500 in 2004 for repeating the offence as a Bolton player at an 11-year-old Middlesbrough supporter, and banned for three matches just weeks later after he spat at Portsmouth defender Arjan de Zeeuw.
Merseyside Police have confirmed they are investigating the incident and spoke to Diouf directly after the game.
The controversial Senegal striker will have to face the music tonight whenhe returns to action against Nottingham Forest in the Carling Cup.
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