Iran Press/ West Asia: Pep Guardiola's side, taking part in the tournament after winning last season's Champions League, made the ideal start as Alvarez struck after 40 seconds, chesting in from close range after Nathan Ake's long-range strike hit the post.
They added a second in the 27th minute when Nino turned Phil Foden's cross into the back of his own net.
England's Foden then got on the scoresheet himself in the second half when he darted through the middle to prod in Alvarez's cross, before the Argentine forward got his second in the 88th minute with a clinical strike into the far corner.
Success meant City emulated Manchester United, Liverpool and Chelsea in lifting the Club World Cup, but became the first English side to win five trophies in a calendar year.
Meanwhile, Egyptian side Al Ahly claimed third place with a 4-2 win against Urawa Red Diamonds of Japan.
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