Iran's human rights record had "degenerated" after the June elections
Many governments have used the internet to curtail freedom of expression at home, the US state department says in its latest annual human rights report.
In many cases new forms of electronic communications are restricted to control domestic dissent, it says.
The wide-ranging report also highlights continuing human rights violations in China against the Uighurs and extra-judicial killings in North Korea.
Iran, Sri Lanka, Burma and Switzerland also come in for criticism.
US Secretary of State Hillary Clinton described the annual country reports - legally required by Congress - as "an important tool in the development of practical and effective human rights strategy by the United States government".
The report said that over the past year many governments had applied "overly broad interpretations of terrorism and emergency powers" as a way of limiting the rights of detainees and other basic human rights.
It said 2009 was a year in which more people gained access to the internet but at the same time governments spent more "time, money and attention" finding ways to control it.
The report said the Chinese government was among the worst offenders for blocking communications.
The government had "increased its efforts to monitor internet use, control content, restrict information, block access to foreign and domestic websites, encourage self-censorship, and punish those who violated regulations", the report said.
Thousands of people at all levels of political life are deployed to monitor electronic communications, it added.
A season of reports from 8-19 March 2010 exploring the extraordinary power of the internet, including:
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"The government at times blocked access to selected sites operated by major foreign news outlets, health organisations, foreign governments, educational institutions, and social networking sites, as well as search engines, that allow rapid communication or organisation of users."
Iran was another country which cracked down on websites such as Facebook and Twitter, the report said.
"Ahead of the June presidential election, on the actual day of election, and during the 27 December Ashura protests, when authorities detained 1,000 individuals and at least eight persons were killed in street clashes, the government blocked access to Facebook, Twitter, and other social networking sites," the report noted.
The report also says that:
• Discrimination against Muslims in Europe is an "increasing concern"
• Sri Lanka violated human rights at the end of last year's campaign against Tamil Tigers and has also curbed press freedom
• Violence and human rights violations in Russia's volatile North Caucasus region have risen dramatically over the past year
• Burma's military junta continued "human rights violations and abuses"
• Cuba "continues to deny its citizens' basic human rights"
The BBC's Kim Ghattas in Washington says the list of countries includes the usual suspects on human rights issues - Iran, China, Burma and North Korea.
But concern about growing anti-Muslim discrimination in Europe is new, she adds.
The report highlighted last year's ban on the construction of minarets in Switzerland as an example.
"Discrimination against Muslims in Europe has been an increasing concern," it said, adding that such developments occurred in countries with "generally strong records" of respecting human rights.
Regarding Sri Lanka, the report accused the government of violating human rights last year as it encircled and defeated Tamil Tiger insurgents.
Switzerland voted in a referendum to ban the building of minarets
"The government's respect for human rights declined as armed conflict reached its conclusion," the report said.
Young Tamil men accounted for an "overwhelming majority" of victims of human rights violations, it said, including extrajudicial killings, even though Tamils only make up about 16% of the population.
The Sri Lankan government has strongly denied any human rights abuses were carried out by its forces.
The report said the military junta in Burma continued its "egregious human rights violations and abuses during the year," including increased military attacks in ethnic minority regions.
And it said Cuba - a country the Obama administration has tried to engage with - continued to deny its citizens' basic human rights, including the right to change their government.
It said Havana had also committed "numerous and serious abuses."