The Pro models were unveiled at Apple's major product event last week alongside new Watches and AirPods, which are also now available. Although many customers order online, shoppers still queued at stores including Apple's Regent Street location in London.
The iPhone 18 Pro brings changes to its camera, processor and battery life. It also carries a higher price, and Apple has indicated that supply might fall short of strong demand.
Apple did not announce a standard iPhone 18 at the event. That model is expected at a separate spring launch under a revised release schedule, rather than arriving alongside the Pro phones.
The iPhone Duo was the event's headline announcement. The device folds in half and marks Apple's entry into foldable phones. Pre-orders are due to open on 16 October, one week before the scheduled Friday, 23 October release.