Freedom House says Internet freedom has declined worldwide in the past year, sparked by broad surveillance, new laws controlling web content, and increasing arrests of social-media users.
However, the U.S.-based watchdog group says activists are becoming more effective in raising awareness of threats.
The findings were released on October 3 in the new edition of the "Freedom of the Net" report that covers developments in 60 countries between May 2012 and April 2013.
The report says Iran, Cuba, and China remain among the most restrictive countries in terms of access to the Internet and censorship.
Adrian Shahbaz, a researcher with Freedom House, told RFE/RL that the death in custody of a blogger in Iran was one of the main reasons why the Islamic republic ended up at the bottom of the list of countries that violate Internet freedom.
"There was actually an instance of an online user being killed in Iran -- that was Sattar Beheshti -- and that was an instance of somebody being brought to jail for something that they had written online, allegedly criticizing the government, and then killed while in police custody," he said.
"So that's something that really affected their score negatively."
Belarus, Pakistan, and Uzbekistan are also among countries with the least Internet freedom, while Azerbaijan, Kazakhstan, Kyrgyzstan, and Russia were found to be partially free.
According to Laura Reed from Freedom House, a number of countries in the former U.S.S.R. have been involved in measures to restrict the Internet.
"In the former Soviet Union over the past year, we've seen additional steps by the governments to block content through a range of legislative and extra-legal measures," she says.
"We've also seen an increase in prosecutions of online users in about half the countries that we cover in the region. Russia, for example, really captures these trends in the past year. So, for example, the number of users that are currently prosecuted for online activities in Russia rose from 31 in 2011 to 103 in 2012."
Russia is singled out in the report as an "important incubator of surveillance technologies and legal practices" that are emulated by other former Soviet republics.
Three former Soviet countries -- Armenia, Georgia, and Ukraine -- have been ranked as free.
The report evaluates each country based on obstacles to access, limits on content, and violations of user rights.
Of the 60 countries surveyed by Freedom House, 34 have experienced drops in their ratings.
Freedom House says that, over the past year, the global number of censored websites has increased, while Internet users in various countries have been arrested, tortured, and killed over their online postings.
The group says Internet freedom also suffered setbacks in several democratic countries, in some cases as governments sought to balance freedom of expression with security.
As an example, Freedom House points to the United States. According to the report, freedom online significantly declined amid revelations of the National Security Agency's extensive surveillance programs.
The report lists the most commonly used methods by governments to control the Internet, including blocking and filtering, cyberattacks against government critics, surveillance, arrests, physical attacks, and murder.
Freedom House says this is the third year in a row that Internet freedom has been in decline globally. Despite the setbacks and negative trends, the report says a perceived increase in civic activism worldwide offers hope for future positive developments.