Elections in 18 separate nations were influenced by online disinformation campaigns last year, suggests research.
Independent watchdog Freedom House looked at how online discourse was influenced by governments, bots and paid opinion formers.
In total, 30 governments were actively engaged in using social media to stifle dissent, said the report.
Educating users to spot fake news and making tech firms police their networks could combat the manipulation, it said.
The annual report studied the state of internet freedom across 65 nations - covering about 87% of the world's net-using population.
For the seventh year running, it said, net freedom had declined as governments stepped up efforts to control what citizens said, did and shared online.
