Superintendent Kevin Smith of Strathclyde police said: "He's the subject of an investigation.
"There have been complaints and TV coverage and we have spoken to two people involved.
"Two people have been arrested but they have now been released."
Houllier said: "I didn't like what he did, it is not acceptable in any way.
"I didn't see it myself but our goalkeeper coach Joe Corrigan said he saw Diouf in the crowd and someone slapped him on the head. Diouf has said sorry to me."
"The player has apologised for his reaction and he realises that his behaviour was totally inappropriate. Whatever the provocation, he shouldn’t have responded the way he did. Liverpool do not condone this action in any way and we will deal with the matter internally.
"It is not conduct we consider acceptable for a Liverpool player and we view the matter extremely seriously. The police have taken some statements, as well as the apology from El-Hadji Diouf."
Kenny Dalglish said on TV: "Diouf's in big trouble."
And all time Red Issue hero, John Aldridge said: "It's a disgrace."
Red Issue could find nobody prepared to blame Chelsea or National front Infiltrators.
Subscribe to the Red Issue magazine on line:
United Kingdom £18.50
"http://www.paypal.com/xclick/business=redissue@ntlworld.com&item_name=10+Issues+United+Kingdom&item_number=UK+10&amount=18.50&no_note=1¤cy_code=GBP"
Europe and Ireland £22
"http://www.paypal.com/xclick/business=redissue@ntlworld.com&item_name=10+Issues+Europe+and+Ireland&item_number=EUR+10&amount=22.50&no_note=1¤cy_code=GBP"
Rest Of World £29
"http://www.paypal.com/xclick/business=redissue@ntlworld.com&item_name=10+Issues+Rest+Of+The+World&item_number=ROW+10&amount=29.00&no_note=1¤cy_code=GBP"