Likely through a scam called "phishing," hackers, apparently based in China, tricked users into revealing personal information, including their e-mail passwords, Google said in a blog post.
"The goal of this effort seems to have been to monitor the contents of these users' e-mails, with the perpetrators apparently using stolen passwords to change peoples' forwarding and delegation settings," the company says.
Google said it recently identified the misuse of the Gmail accounts, many of which belonged to "senior U.S. government officials, Chinese political activists, officials in several Asian countries [predominantly South Korea], military personnel, and journalists."
The company said it has notified victims as well as government authorities and stressed that the attack did not come from a breach of Google's e-mail system.
"But we believe that being open about these security issues helps users better protect their information online," Google said.
Sony is still reeling from cyberattacks that compromised millions of user accounts, and data-breach issues are set to make another appearance at a congressional hearing on Thursday.