As Vladimir Putin starts his fourth Kremlin term, authorities are turning up the heat on popular websites and apps ostensibly to fight terrorism but analysts say the real motive is to muzzle critics.

A move this week to block the strongly encrypted messenger Telegram, less than a month after Putin's crushing poll win, marks a new stage in the crackdown launched after his previous victory in 2012.

Telegram, which has 200 million users and is ironically the go-to messaging app for top Kremlin officials, was specifically designed by Russian developers to circumvent the Kremlin's security forces.

Putin has gradually brought media, primarily television, under state control since the early 2000s.

Experts say the Kremlin recognises the internet as the principal threat to its domination and one of the last refuges of free speech -- especially after it helped fuel unprecedented mass demonstrations when Putin returned to the presidency six years ago after four years as prime minister.

"The Kremlin got scared and responded with an attack on internet freedoms," said Andrei Soldatov, editor-in-chief of Agentura.ru, a site monitoring the security services.

In the summer of 2012, Russia created a blacklist of sites showing child pornography or drug use and also those deemed to be "extremist" -- a term vague enough to include opposition activism. The professed intention of the move was to protect children from harmful content online.

Tens of thousands have taken to the streets to stage anti-Putin protests Maxim ZMEYEV, AFP/File

Two years later, the parliament unleashed a barrage of new anti-terrorist laws, including a ruling that blogs with more than 3,000 viewers per day must face the same strict regulations as news media.

Since then, Russian and foreign internet providers have been legally obliged to store the data of their Russian users in Russia.

This led to the blocking of professional networking site LinkedIn, which did not comply.

Subsequently, new legislation citing terror threats has forced all "distributors of information" -- including boggers and even social media platform VK (formerly VKontakte), its owner Mail.ru, and internet giant Yandex -- to retain all user data for six months and provide it to the authorities on request.

Under the latest measure imposed late last year, the authorities are able ban VPN services that allow users to bypass Russian site blocks by simulating a connection from another country.

- 'Spread fear' -

This legislative onslaught has been widely used against the opposition, who are ignored by mainstream news media but are active online.

Rights groups have also been hit hard.

The blog and website of the main opposition figure, Alexei Navalny, have been blocked partially or completely several times over his calls for street protests or exposes of official corruption.

Sites used by the opposition organisation of former oil tycoon Mikhail Khodorkovsky, who now lives in London, were also blocked after they were designated "undesirable" -- a new term for foreign entities that has also been used against the foundation of the US financier George Soros.

"The purpose is to spread fear, (to) make people think that the state controls everything and that you can't hide anywhere, that all data can be collected," said Sarkis Darbinyan, a lawyer and director of a digital rights centre in Moscow.

Tin mugs depicting Putin behind an interdiction sign in an internet shop run by opposition leader Alexey Navalny Mladen ANTONOV, AFP/File

But Russia cannot simply impose a local version of China's "Great Firewall" cutting off all access to sites, he said.

"Unlike China, where the internet was constrained since the beginning, Russian internet started off as very free," he said.

Ultimately global players like Facebook, Twitter, Google, WhatsApp and Telegram who want to operate in Russia will have to comply with state restrictions or get blocked, he said, adding it's only "a matter of time."

Navalny's anti-corruption investigations have got millions of views on YouTube and social media, and last year helped mobilise tens of thousands to take to the streets in anti-Kremlin protests. Most of the demonstrators were young people who communicate online.

Opposition figures "find new ways of working: they go over to cloud services, widely use social media such as Facebook and Twitter, and inform people how to get round blocks," said Artyom Kozlyuk, director of internet rights NGO Roskomsvoboda.

But he said he had already observed a "slow process of enslavement" among internet users with many realising "that it's better not to publish anything risky so as to avoid the attention of special services and prosecutors."