The confusion arose from the fact that the Department of Telecom issued the order to internet service providers to block the websites on Friday evening. Since their technical staff were not at full strength over the weekend, the providers could only implement the order patchily. Until Monday morning, that is, when black pages greeted users.

"Free and open access to porn websites has been brought under check," NN Kaul, a spokesman at the department of telecommunications told Reuters.

"We don't want them to become a social nuisance."

The ban should not have come as a total surprise as it has been mulling how to curb online pornography, particularly child pornography, for over a year.

Late last year, a meeting was held to see if the government had the technical capacity to ban sites hosted by servers based abroad. A tentative list of unacceptable websites was drawn up.

In the meantime, a lawyer filed a private petition in the Supreme Court asking for a total porn ban on the grounds that it provoked violence against women. On July 7, the Supreme Court rejected this petition, saying that such a ban would infringe the personal liberty of Indians to watch what they liked in the privacy of their homes.

However, in their observations, the judges remarked that the Home Ministry, despite its professed concern, had been "lethargic" in blocking child pornography sites, a remark that experts say may have spurred the weekend crackdown.

It's not clear which sites are on the list of 857 or how long the block will last. The list has not been made public.

The block had already incensed users on Twitter and is bound to revive a wider, classical debate between those who wish to ban pornography and those who regard it as their inalienable right to watch it. 

"The government needs to be more realistic. Countries like China have tried and failed to ban pornography. People start using proxy servers. And all a ban does is act as a catalyst for more traffic to that site. Bans excite curiosity," said Pavan Duggal, a Supreme Court lawyer in New Delhi who specialises in cyber security.

Mr Duggal pointed out that the government action was to block access rather than ban the websites.

The first simply requires internet service providers to block access to such sites in India whereas a ban has wider import – if anyone tries to access a banned website, it is considered a crime.

Last year, data provided by the world's largest porn website, PornHub, showed that India is the fifth largest consumer of online pornography in the world (with Saturday being the most popular day). The same data showed that nearly one-fourth of the site users from India are female.

What's striking about this consumption is that much of it is on mobile phones since millions of Indians have no computer with an internet connection at home, but do possess smartphones. Nearly 50 per cent of porn consumers in India use their smartphones.

For poor Indians who don't have smartphones, street vendors in local bazaars can help. These vendors download porn clips onto their laptops and then transfer them to the mobile phone owner's memory cards for a few rupees in the hugely popular phenomenon known as "mobile porn".

One Twitter user said the block was the government's only way to "stop its own MPs in Parliament" watching porn.

Another lamented the multiple bans in India: "Ban on tobacco, ban on drinking under 25, ban on beef-eating, and now a ban on porn. This is Sparta!"

More sanguine souls wrote: "Look on the bright side.  Your 10GB data plan will last longer' and 'Real men don't need porn – we have our imagination."

In 2011, India urged social networks to screen content and remove offensive material. A year later, the government faced criticism for ordering dozens of Twitter accounts to be blocked for spreading rumours.

Censorship extends to Bollywood and Hollywood movies, with regulators often forcing producers to edit dialogues and scenes to suit audiences in the generally conservative country.

Mr Kaul said the government was working on a long-term solution and suggested the ban would not remain indefinitely.

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with Reuters