Countries that build electronic barriers to parts of the internet or filtered search engine results contravene the UN's Universal Declaration on Human Rights, which guarantees freedom of information, she said.
Describing the growing internet curbs as the modern equivalent of the Berlin Wall, she said: "A new information curtain is descending across much of the world."
Addressing concerns about cyber spying in China that have prompted Google Inc, the internet search company, to threaten to quit that market, Clinton said: "Countries or individuals that engage in cyber attacks should face consequences and international condemnation.
"In an interconnected world, an attack on one nation's networks can be an attack on all."
Google vs China
Google has said it may shut its Chinese-language Google.cn website and offices in China after a cyber attack originating from China that also targeted others.
The company has said it no longer wants to censor its Chinese Google.cn site and wants to talk with Beijing about offering a legal, unfiltered Chinese site. Searches for sensitive topics on Google.cn are still largely being censored.
Many in China see Google's ultimatum as a business tactic because its market share trails the popular Chinese search site Baidu, which is strictly censored.
Despite extensive public debate of the Google issue in China, hacking has been rarely mentioned in official media.
Facebook, Twitter and YouTube are blocked in China.
Clinton noted that text messages had helped rescuers in Haiti find a young girl and two women trapped in a supermarket after the earthquake and the US text "HAITI" campaign that had raised more than $25 million from mobile phone users.
But she cautioned that internet technologies were a mixed blessing because along with the benefits of spreading knowledge and empowering citizens, the web is used by al-Qaeda to spread hatred and by authoritarian states to crush dissent.
"The same networks that help organise movements for freedom also enable al-Qaeda to spew hatred and incite violence against the innocent," she said.
"And technologies with the potential to open up access to government and promote transparency can also be hijacked by governments to crush dissent and deny human rights."
China, Tunisia and Uzbekistan have stepped up censorship of the internet, while Vietnam has cut access to popular social networking sites and Egypt has detained 30 bloggers and political activists, Clinton said.
Saudi Arabia, China and Vietnam have also blocked internet access to religious information or silenced people of faith, she said.
The US recognised limits to freedom of speech and the need to combat use of the internet to spread hate speech, recruit terrorists or distribute stolen intellectual property, Clinton said.
"But these challenges must not become an excuse for governments to systematically violate the rights and privacy of those who use the Internet for peaceful political purposes," she said.