Arsene Wenger: "They are one of the teams to beat, but not more than any of the other ones we have played until now.
"We have played Everton, who were as good as City, and Southampton, who were as good as City."
City's prolific strikeforce of Sergio Aguero and Alvaro Negredo put them ahead before Fernandinho struck twice and David Silva and Yaya Toure also scored.
Arsenal remained in contention for over an hour after two goals from Theo Walcott but Mertesacker's late header was mere consolation.