Per Sky Sports' Lyall Thomas, the Metropolitan Police has confirmed the arrest of a single individual who threw a lighter on the pitch. He remains in custody pending further enquiries.
Per Thomas, Chelsea spokesman Steve Atkins made it clear the club plans to ban whoever is responsible:
We condemn such idiotic and dangerous behaviour and if we can identify those responsible then we will ban them from Stamford Bridge.
It's worth noting that it's a criminal offence too and we will support a police prosecution if we can identify, along with the police, those individuals responsible for that behaviour today.
The incident occurred when City youngster David Faupala cancelled out Diego Costa's goal for the home side in the first half. BBC reporter Jason Mohammad provided details of the incident in footage relayed by the Match of the Day Twitter account:
Simon Johnson of the London Evening Standard was among those quick to condemn the behaviour:
This marks the second time this weekend an FA Cup fixture has been marred by this type of incident. On Saturday, West Bromwich Albion midfield man Chris Brunt was struck by a coin allegedly thrown by one of his own fans following the Baggies' 3-1 away defeat at Reading.
BBC Sport Football Focus host Dan Walker reported the incident:
That prompted the FA to announce an immediate investigation:
Penalties from the clubs and football's governing body should be severe to deter these kinds of actions in the future. Both institutions must prevent this behaviour from becoming a malicious trend that serves as a stain on the sport and its most revered competitions.