Google said its security was not breached but indicated individuals' passwords were obtained through fraud.
Chinese political activists and officials in other Asian countries were also targeted, Google said.
"Google detected and has disrupted this campaign to take users' passwords and monitor their emails," the company said on Wednesday.
"We have notified victims and secured their accounts. In addition, we have notified relevant government authorities."
The e-mail scam uses a practice known as "spear phishing" in which specific e-mail users are tricked into divulging their login credentials to a web page that resembles Google's Gmail web service (or which appears related to the target's work) but is in fact run by hackers, according to a technical report released by Google.
Having obtained the user's e-mail login and password, the hackers then tell Gmail's service to forward incoming e-mail to another account set up by the hacker.
In Washington, the BBC's Adam Brookes says it is extremely difficult for analysts to determine whether governments or individuals are responsible for such attacks.
But the fact that the victims were people with access to sensitive, even secret information, raises the possibility that this was cyber espionage, not cyber crime, our correspondent says.