Iraqi security forces in a western province unaffected by anti-government protests are detaining people for posting messages of solidarity with the rallies, Human Rights Watch said on Monday.
At least two people have been arrested and a third interrogated in Anbar province, a Sunni-majority desert region in the country's west, after Facebook posts.
Protests demanding an overhaul of the political system have rocked Baghdad and southern Iraq, both mainly Shiite, but have not reached the west or Kurdish-majority north.
Residents of western Iraq have told AFP and HRW they were remaining quiet of out of fear of being accused of being "terrorists" or backers of ex-dictator Saddam Hussein.
But security forces there appear to be monitoring social media accounts, HRW said on Monday.
"Despite years of terrible conflict, many Iraqis have felt free to speak out on political issues," said HRW's Middle East director Sarah Leah Whitson.
"But these cases mark a disturbing change, if you contrast these men's entirely peaceful political statements with the completely inappropriate response by the Anbar authorities."