Many of the over 1.5 billion WhatsApp users, both on Android and iOS, were unable to log into the app.
WhatsApp services in Europe, India, Malaysia appear to have been badly hit and even parts of the US and Africa appear to have been affected.
WhatsApp, a messaging service owned by Facebook, said some users were still facing issues while accessing its messaging service.
WhatsApp founder Jan Koum has said that India is an important market, if the app aims to hit the one billion mark.
WhatsApp founder Jan Koum on Sunday issued an apology and blamed a network router for Saturday's outage of the mobile messaging app.
WhatsApp, the rapidly expanding mobile messaging app, suffered an outage for more than three hours on Saturday.