Restrictions on "torrent sites" that offer free movie and music downloads are routine in India to prevent copyright infringement. Pornography websites are also blocked by court orders seeking to protect children.
But in recent months, websites such as the discussion board Reddit, messaging service Telegram and comedy site College Humor have been blocked for intermittent periods, often for days and only in some regions, baffling internet users.
"It's not making any sense, what's happening," said Apar Gupta, executive director at the non-profit Internet Freedom Foundation (IFF). "A lot of these blocks are also happening in such a way that no notices are displayed." Since January, there have been at least 250 reports of websites blocked on networks operated by Jio, a unit of Reliance Industries, Bharti Airtel and Hathway, the IFF said in a letter to the telecoms department.
Some internet users have posted on social media screenshots of pages displaying messages saying a website was blocked to comply with government orders.
When Reuters tried to access CollegeHumor.com on Wednesday a message read: "Your requested URL has been blocked as per the directions received from Department of Telecommunications, Government of India."
An official at the telecoms department, which last year approved rules on net neutrality--the concept that all websites and data on the Internet be treated equally--declined to comment when contacted by Reuters.
Complaints by Indian internet users have covered "most forms of net neutrality violations," IFF's Gupta said.
Nearly 60 per cent of the user reports compiled by the foundation since January involved Jio networks, the IFF's data showed.
A Jio representative did not respond to an emailed request for comment. Hathway did not reply to phone and email requests for comment.
Bharti Airtel said in a statement it "supports an open internet" and does not block content unless directed by authorities. It did not say if it was currently blocking any websites.
Reddit did not respond to an emailed request for comment outside regular U.S. business hours.
Telegram, which has been blocked previously in Russia and Iran, did not immediately respond to a phone message seeking comment.
If websites are blocked based on government or court orders, or internet firms have legal grounds to restrict web pages, they might not violate net neutrality rules, said Gurshabad Grover, a researcher at the non-profit Centre for Internet and Society.
"But in this case we're not entirely sure," he said.
Other sites blocked this year include tax portal Taxscan and legal database Indian Kanoon.
After complaints from Jio's internet users, Indian Kanoon founder Sushant Sharma said he had been told by Jio the portal was blocked for one day last week due to a government order.
"By evening, apparently, that order was taken back," said Sharma, whose website has some 150,000 daily visitors.