Online freedom has suffered setbacks in many countries, but also some gains, amid the Arab Spring uprisings and political upheaval in parts of the world, a new study showed.

The report by the research group Freedom House found that 20 countries "experienced a negative trajectory since January 2011" as authorities used newer, more sophisticated controls to quell dissent on the Internet.

"The findings clearly show that threats to Internet freedom are becoming more diverse," said Sanja Kelly, project director at Freedom House and co-author of the report released Monday covering the period from January 2011 to May 2012.

"As authoritarian rulers see that blocked websites and high-profile arrests draw local and international condemnation, they are turning to murkier -- but no less dangerous -- methods for controlling online conversations."

The study found that Estonia had the highest level of online freedom among the 47 countries examined, while the United States ranked second.

Iran, Cuba, and China received the lowest scores and 10 other countries received a ranking of "not free" -- Belarus, Saudi Arabia, Uzbekistan, Pakistan, Thailand, Vietnam, Myanmar, Ethiopia, Bahrain and Syria.

The worst declines, according to the report, were in Bahrain, Egypt, and Jordan, reflecting "intensified censorship, arrests, and violence against bloggers."

It said online freedom was also hurt in Mexico "in the context of increasing threats of violence from organized crime," and in Ethiopia, "possibly reflecting a government effort to establish more sophisticated controls before allowing access to expand."

In Pakistan, the downgrade "reflected extreme punishments meted out for dissemination of allegedly blasphemous messages" and tighter censorship by regulators.

Improvements were cited in 14 countries, including some with "a dramatic regime change or political opening" such as Tunisia, Libya and Myanmar. But restrictions also eased in some other countries such as Georgia, Kenya, and Indonesia, where the report cited "a growing diversity of content and fewer cases of arrest or censorship than in previous years."

Fourteen countries were listed as "free," and 20 were labeled "partly free" in the report.

The report said China, which has the world's largest population of Internet users, also has "the most advanced system of controls" and that it has become "even more restrictive."

It cited the 2011 detainment of dozens of activists and bloggers, who were held incommunicado for weeks before several were sentenced to prison.

The Beijing government "tightened controls over popular domestic microblogging platforms, pressuring key firms to more stringently censor political content and to register their users' real names," the report said.

It added that China appeared to be "an incubator for sophisticated restrictions," with governments such as Belarus, Uzbekistan, and Iran using China as a model for their own Internet controls.