The watchdog's "freedom on the net" report showed that online freedom declined in 33 of the 65 countries assessed, while improvements were measured in 16 countries.
Governments around the world are increasingly using social media to manipulate elections and monitor their citizens in a worrisome trend for democracy, a human rights watchdog said on Tuesday.
An annual report on online freedom by the nonprofit group Freedom House found evidence of "advanced social media surveillance programs" in at least 40 of 65 countries analysed.
The report said global internet freedom declined for a ninth consecutive year, as authorities in some countries simply cut off internet access as part of their manipulation efforts, while others employed propaganda armies to distort information on social platforms.
"Many governments are finding that on social media, propaganda works better than censorship," said Mike Abramowitz, president of Freedom House.
"Authoritarians and populists around the globe are exploiting both human nature and computer algorithms to conquer the ballot box, running roughshod over rules designed to ensure free and fair elections."
Disinformation was the most commonly used tactic to undermine elections, according to the group.
"Populists and far-right leaders have grown adept not only at creating viral disinformation but also at harnessing networks that disseminate it," said the watchdog's 2019 "freedom on the net" report.
The researchers said that in 47 out of the 65 countries, individuals were arrested for political, social or religious speech online and people were subjected to physical violence for their online activities in at least 31 countries.
China remained the world's worst abuser of internet freedom for the fourth consecutive year as the government stepped up information controls amid protests in Hong Kong and ahead of the 30th anniversary of the Tiananmen Square massacre, the report said.
Online freedom declined in 33 of the 65 countries assessed, including the US, the survey found.
In the US, "law enforcement and immigration agencies expanded their surveillance of the public, eschewing oversight, transparency, and accountability mechanisms that might restrain their actions", Freedom House said.
"Officials increasingly monitored social media platforms and conducted warrantless searches of travellers' electronic devices to glean information about constitutionally protected activities such as peaceful protests and critical reporting."
The report said disinformation was rampant in the US, focusing on the November 2018 midterm elections, and that "both domestic and foreign actors manipulated content for political purposes, undermining the democratic process and stoking divisions in American society".
The biggest declines were in Sudan and Kazakhstan, followed by Brazil, Bangladesh and Zimbabwe. Improvements were measured in 16 countries, with Ethiopia recording the largest gains.
Despite the grim outlook, Abramowitz cited some positive examples of technology spurring democratic change, including in Lebanon, where people "are rallying their fellow citizens" for reforms.