Microsoft offered up its own Web-based alternatives, saying for instance that users of its free email service, Hotmail, don't have to worry about the content of their emails being used to serve up ads.
The attack ads appeared in papers including USA Today, The Wall Street Journal and New York Times. They followed an overhaul of the way that Google handles user data, which the company announced last week. The aim is to streamline more than 70 privacy policies into one main document plus about a dozen others.
The change will allow Google to share user data across services like Gmail, YouTube and its social network, Google Plus, increasing the ability of advertisers to focus their message and target relevant audiences.
"Every data point Google collects and connects to you increases how valuable you are to an advertiser," Microsoft says in the ad.
The company does not dispute that it serves up ads based on words in private emails written by users of Gmail, but says such scanning is automated and is similar to how many email providers filter out spam. It has operated that way since Gmail's introduction in 2004.
Both companies offer several controls to prevent advertisers from tracking users' online activity.
He said Microsoft is in no position to point fingers, since it also collects a lot of user data from its search engine, Bing, and will adjust search results based on information it finds in users' Facebook accounts if they are logged in.
"I think they're largely about the same," Sullivan said. "It would not be hard to go through and pick any major Internet company, talk about the kind of data they collect and start getting people paranoid."
Explore further: Google to merge user data across more services