A man has been arrested in connection with allegations of racist abuse aimed at Leeds striker El-Hadji Diouf by Millwall fans, Scotland Yard said on Tuesday.
The 53-year-old man was arrested after Sky Sports News broadcast footage on Monday night which appeared to show some of the London club's fans hurling insults at the Senegalese forward during a game at the Den in November.
Millwall said they were "appalled'' by the footage and vowed to reopen an investigation into allegations of racial abuse, which initially emerged online.
Club captain Danny Shittu said he was "deeply saddened'' by what he had seen in the documentary.
The man, from Enfield, north London, remained in custody on Tuesday night on suspicion of a racially aggravated public order offence, a Metropolitan Police spokesman said.
Millwall have promised to ban any individuals for life if they are found guilty.
The footage appears to show fans hurling insults at Diouf during a Championship clash between the two clubs at the south-east London ground on November 18.
The Lions' chief executive, Andy Ambler, said that an initial investigation was closed after Diouf said he heard no racist abuse aimed at him.
"Like all Millwall fans and football fans generally, we're appalled by the footage we've seen,'' he said.
"Sadly racism exists in our society and certain individuals want to bring it into football grounds. It's down to us in football do to more.
Shittu later released a statement on the club website which read: "I am deeply saddened by the documentary on Sky Sports showing racism by the minority at matches including at Millwall FC.
"As the current Millwall captain, racism is something I do not stand for or condone.
"I am currently in talks with the club about any further measures we can take in dealing with and eradicating racism at matches.
"I do not want the events shown yesterday to tarnish the hard work done by the club thus far and their efforts in tackling this issue.
"Every player at all levels has the right to represent their club without being subjected to homophobic or racial taunts. I will do all I can to help eradicate racism at my club and in football.''
Bolton forward Marvin Sordell has previously made allegations of racial abuse towards Millwall's fans.
A banner stating "Sordell is a c***'' was unfurled at a later game which came in the wake of his claims, while a 13-year-old boy was banned from the ground.
Information from the Press Association was used in this report.