Chelsea and the Metropolitan Police are investigating after an incident of alleged racist abuse was highlighted on social media on Saturday.
Twitter users highlighted a first-half incident when a supporter appeared to abuse Manchester City forward Raheem Sterling.
A Chelsea spokesman said: 'We're aware of reports and video footage. We will investigate the matter and take the strongest possible action where necessary.'
The Met Police said: 'We are aware of a video circulating online in which it is claimed racial abuse was allegedly directed at a player at a Chelsea v Manchester City game at Stamford Bridge on Saturday, December 8.