Google says a "majority" of users were affected.
It's not immediately clear what caused the disruption. Google did not immediately return messages left by The Associated Press.
There were signs of troubles Monday, but it's not clear whether they are related to the more severe outages that began Tuesday afternoon.
Google Inc. said in a posting on its Web site that service has been restored to some users. The company says it expects to have the problem fixed later Tuesday.
Though occasional disruptions are common, widespread outages involving Google's services are rare. They are becoming a bigger threat to Google as it tries to sell more of its services to businesses.