SAN FRANCISCO - Google threatened Tuesday to shut down its operations in China after uncovering what it said were "highly sophisticated" cyberattacks aimed at Chinese human rights activists.

Google said China-based cyber spies struck the Internet giant and at least 20 other unidentified firms in an apparent bid to hack into the email accounts of activists around the world.

Google said the online espionage has it reconsidering its business operations in China and it has decided to no longer filter Internet search engine results in China.

"These attacks and the surveillance they have uncovered - combined with the attempts over the past year to further limit free speech on the Web - have led us to conclude that we should review the feasibility of our business operations in China," Google chief legal officer David Drummond said in a blog post.

"We have decided we are no longer willing to continue censoring our results on Google.cn, and so over the next few weeks we will be discussing with the Chinese government the basis on which we could operate an unfiltered search engine within the law, if at all," he said.

Drummond said Google realizes that defying Chinese government demands regarding filtering Internet search engine results may mean having to shut down its operations in China.

Drummond said Google detected in a mid-December "a highly sophisticated and targeted attack on our corporate infrastructure originating from China that resulted in the theft of intellectual property from Google."

Evidence indicated that the attackers were trying to get access to Gmail accounts of Chinese human rights activists, according to Drummond.

Google believes the attack was mostly blocked and that only minor information, like creation dates and subject lines, were stolen from two accounts.

At least 20 other large companies including finance, Internet, media, technology, and chemical businesses were similarly attacked, according to Google.

"We are currently in the process of notifying those companies, and we are also working with the relevant US authorities," Drummond said.

Google said its investigation revealed that accounts of China human rights activists who use Gmail in Europe, China, or the United States have been "routinely accessed" using malware sneaked onto their computers.

"Independent of the attack on Google, we have discovered that the accounts of dozens of US-, China- and Europe-based Gmail users who are advocates of human rights in China appear to have been routinely accessed by third parties," Drummond said.

"We have taken the unusual step of sharing information about these attacks with a broad audience not just because of the security and human rights implications of what we have unearthed, but also because this information goes to the heart of a much bigger global debate about freedom of speech," he said.

"The decision to review our business operations in China has been incredibly hard, and we know that it will have potentially far-reaching consequences," he added.

Google was careful to stress that the decision was made by the California company's executives in the United States and not by workers within easy reach of authorities in China.

Google also said it has used information gained from studying the attack to improve the Internet titan's security.

"We have been working hard to secure our systems, confirm the facts, and notify the relevant authorities," said Google spokesman Gabriel Stricker.

"We've gone public with this as quickly as we sensibly could."