The 53-year-old man was arrested after Sky Sports News broadcast footage which appeared to show some Millwall fans hurling insults at the Senegalese forward during a game at the Den in November.
Millwall said they were appalled by the footage. The club captain, Danny Shittu, said he was deeply saddened by what he had seen in the documentary, reports The Guardian.
The man, from Enfield, north London, was in custody on suspicion of a racially aggravated public order offence, a Met spokesman said.
Millwall have promised to ban any individuals for life if they are found guilty.