Adults will still be able to access the sites using virtual private networks (VPNs) or proxy servers.
In July, the Supreme Court expressed its unhappiness over the government's inability to block sites, especially those featuring child pornography.
Telecom companies have said they will not be able to enforce the "ban" immediately.
"We have to block each site one by one and it will take a few days for all service providers to block all the sites," an unnamed telecom company executive told The Times of India newspaper.
A senior official, who preferred to remained unnamed, told the BBC Hindi that India's department of telecommunications had "advised" telecom operators and Internet service providers to "control free and open access" to 857 porn sites.
"There is no total ban. This was done in the backdrop of Supreme Court's observation on children having free access to porn sites. The idea is also to protect India's cultural fabric. This will not prevent adults from visiting porn sites," the official said.
"It is a temporary arrangement. The telecommunications depart will eventually come up with a long term policy [of controlling access to these sites later," he said.
In July, the top court had observed that it was not for the court to order a ban on porn sites.
"It is an issue for the government to deal with. Can we pass an interim order directing blocking of all adult websites? And let us keep in mind the possible contention of a person who could ask what crime have I committed by browsing adult websites in private within the four walls of my house. Could he not argue about his right to freedom to do something within the four walls of his house without violating any law?," the court said.
According to statistics released by adult site Pornhub, India was its fourth largest source of traffic in 2014, behind the US, UK and Canada.
Pranesh Prakash of the Bangalore based Centre for Internet and Society said the directive to block the 857 sites was "the largest single order" in India.
"The government's reasoning that it is not a ban because adults can still access the porn sites is ridiculous," he told the BBC.
The move has caused a great deal of comment on Indian social media networks, with many prominent personalities coming forward to condemn it.
Popular author Chetan Bhagat, writer and commentator Nilanjana Roy, politician Milind Deora and director Ram Gopal Varma have all added their voices to the debate.
The hashtag #pornban was also picking up momentum in India, with hundreds of tweets on the subject.
It's not all outrage though. A number of people have been vociferously defending the move, saying that porn is an assault on Indian culture.
And according to some tongue in cheek observers, one bit of good that may come of the ban, is that the necessity of having to use proxy servers would inadvertently force Indians to take their privacy seriously.
"I request the powers that be to therefore lose no time in introducing web filtering to India, so that Indians can truly enjoy the free internet, without restrictions and fear", writer Shivam Vij said on Indian news website Scroll.

The world governing body said local officials could instead be appointed by the Pakistan Cricket Board.
There will be two Twenty20 and three one-day internationals from 22-31 May.
On Thursday Zimbabwe cancelled the tour after news of a terrorist attack in Karachi, then retracted the statement.
No top-level international cricket has been played in Pakistan since Sri Lanka's team bus was attacked there by gunmen in 2009.
India has blocked free access to 857 porn sites in what it says is a move to prevent children from accessing them.
[NEXT_CONCEPT]
The International Cricket Council will not send umpires to Pakistan to officiate in their upcoming limited-overs series against Zimbabwe because of security concerns.