By Jonathan Fildes
Technology reporter, BBC News
More than 30,000 account details have been posted online
Google's web-based e-mail system, Gmail, has been targeted as part of an "industry-wide phishing scheme".
The firm said that it had immediately safeguarded the affected accounts.
BBC News has seen two lists that detail more than 30,000 names and passwords from e-mail providers, including Yahoo and AOL, which were posted online.
The lists also include details of thousands of Microsoft Hotmail users. Google said fewer than 500 of its accounts had been affected by the scam.
However, the search giant revealed that it had discovered a third list, but would not say how many accounts it showed.
Phishing involves using fake websites to lure people into revealing data such as bank account details or login names.
"We recently became aware of an industry-wide phishing scheme through which hackers gained user credentials for web-based mail accounts including Gmail accounts," said a Google spokesperson.
"As soon as we learned of the attack, we forced password resets on the affected accounts. We will continue to force password resets on additional accounts when we become aware of them."
The firm stressed that the scam was "not a breach of Gmail security" but rather "a scam to get users to give away their personal information to hackers".
The phishing scam was originally thought to target just Hotmail users.
It was brought to light when 10,000 Hotmail addresses were posted online at Pastebin, a website commonly used by developers to share code.
The list was reported by technology blog Neowin.
However, a second list of 20,000 names has since emerged containing e-mail addresses and passwords from Hotmail, Yahoo, AOL, Gmail and other service providers. A third list, which has not been seen by the BBC, was discovered by Google.
Some of the accounts on the list of 20,000 names appear to be old, unused or fake. However, BBC News confirmed that many - including Gmail, Yahoo and Hotmail addresses - were genuine.
Other addresses on the list include Comcast and Earthlink accounts.
It is not clear whether the new lists was part of the same phishing attack that collected the Hotmail addresses or a separate scam.
A spokesperson for Microsoft said phishing was an "industry-wide problem".
"Our guidance to customers is to exercise extreme caution when opening unsolicited attachments and links from both known and unknown sources, and that they install and regularly update their anti-virus software."
Both lists can still be accessed online.
A spokesperson for Yahoo urged consumers to "take measures to secure their accounts whenever possible, including changing their passwords".
Carole Theriault of security firm Sophos agreed.
"Getting access to one password can give someone access to lots of things," she said.
People should change their password on any other site where they use it, she added.
A recent report by the firm said that around 40% of people had the same password for every website they used.
"People need to see a difference between an online bank account and booking cinema tickets online," she told BBC News.
But, she said, blame did not rest with the users of the e-mail services, who most likely clicked on a link in a scam message.
"Phishing attacks are very subtle these days," she said. "People do all kinds of tricky things."
Fake websites, which ask for a user's login details, can be made to look like those of reputable companies.
"This should be a wake-up call to Google and Microsoft to educate their users," said Ms Theriault.