Iran authorities blocked access to Instagram and the Telegram messaging app

Google has faced calls to lift restrictions on its services for Iranian internet users

It comes as the death toll from violent anti-government protests climbed to 21

Iran has shut down social media in an attempt to stop unrest from spreading widely as deadly anti-government protests continue across the country.

Authorities have blocked access to Instagram and the Telegram messaging app as part of a clamp down on its citizens' internet communications.

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Meanwhile, Google has faced calls to lift restrictions on its services for internet users in Iran so that millions of protesters can 'connect and organise'.

Iran has shut down social media in an attempt to stop unrest from spreading widely as deadly anti-government protests continue across the country. An iranian man is pictured showing how one of his apps is no longer functioning

President Hassan Rouhani has insisted people are 'absolutely free' to express their anger but 'criticism is different to violence and destroying public property'

President Hassan Rouhani has insisted people are 'absolutely free' to express their anger but 'criticism is different to violence and destroying public property.'

But the demonstrations, which have claimed 21 lives and led to 450 arrests so far, were fanned in part by messages sent on social media platforms prompting a black out of some services on Sunday.

Telegram in particular is very popular in Iran, with more than 50 per cent of the country's 80m population said to be active on the app.

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Iran state TV website reported the decision citing an anonymous source who said it was 'in line with maintaining peace and security of the citizens.'

The source said: 'With a decision by the Supreme National Security Council, activities of Telegram and Instagram are temporarily limited.'

Google meanwhile has been urged to lift internet restrictions in the country.

Dr Steven Murdoch, a security researcher in the Computer Science Department, University College London, told Sky News that Google blocks users from Iran from accessing many of its services because of US sanctions.

The demonstrations, which have claimed 21 lives and led to 450 arrests so far, were fanned in part by messages sent on social media platforms prompting a black out of some services on Sunday

But as a result, people have encountered difficulties trying to use counter-censorship apps such as Signal, which was set up to bypass blocking by disguising itself amongst Google's services.

NSA whistleblower Edward Snowden later tweeted: 'Many US politicians say they want to help Iranian protesters. If they're serious, one phone call could get @Google to restore millions of protesters' ability to connect and organize.'

Google has not yet responded to requests for comments, Sky said.

Meanwhile Iran's supreme leader has blamed the country's 'enemies' on Tuesday for days of unrest that have seen 21 killed and hundreds arrested in the biggest test for the Islamic regime in years.

The foreign ministry in Tehran also lashed out at US President Donald Trump after his latest Twitter attack on the Iranian authorities over the protests, insisting he should focus on 'homeless and hungry people' in his own country.

In a speech carried on state television Ayatollah Ali Khamenei broke his silence on the protests for the first time since they erupted last Thursday.

'The enemies have united and are using all their means, money, weapons, policies and security services to create problems for the Islamic regime,' the supreme leader said.

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'The enemy is always looking for an opportunity and any crevice to infiltrate and strike the Iranian nation.'

A fifth night of unrest Monday to Tuesday saw six protesters killed during an attack on a police station in Qahderijan in the central province of Isfahan, state TV said Tuesday.

At least three other towns near the cultural hub of Isfahan also saw violence overnight, causing the deaths of a young member of the Revolutionary Guards, a policeman and a bystander.

Ayatollah Ali Khamenei (centre) has blamed the country's 'enemies' on Tuesday for days of unrest that have seen 21 killed and hundreds arrested in the biggest test for the Islamic regime in years

The estimated death toll is now 21 since protests began in second city Mashhad and quickly spread to become the biggest challenge to the Islamic regime since mass demonstrations in 2009.

As violence has grown, authorities have stepped up arrests, with at least 450 people detained in Tehran since Saturday and 100 more around Isfahan on Monday, officials told local media.

US President Donald Trump, who has repeatedly criticised Tehran since the latest protests began, praised the demonstrators for acting against the 'brutal and corrupt' regime and said Iranians had 'little food, big inflation and no human rights'.

Iran's foreign ministry fired back that the US leader was 'wasting his time sending useless and insulting tweets' and advised him to pay more attention to 'domestic issues' in his homeland.

The unrest in Iran appears leaderless and focused on provincial towns and cities, with only small and sporadic protests in Tehran as a heavy police presence was reported.

Ali Shamkhani, secretary of the Supreme National Security Council, described the unrest as a 'proxy war against the Iranian people' and said online accounts in the United States, Britain and Saudi Arabia are fomenting protests.

A Revolutionary Guards spokesman said the powerful force had not been requested to intervene directly, but insisted officials will 'take decisions to finish' insecurity if it persists.

Iran's reformist politicians condemned the violence and blamed the US for encouraging 'troublemakers', but also called on the authorities to address economic grievances that have fuelled the protests.

Protesters clashed with security forces pver the new year as they tried to storm a police station in Qahderijan, a town of 30,000 in the Isfahan region of central Iran

'Without doubt the Iranian people are confronted with difficulties in their daily lives... and have the right to peacefully demand and protest,' said a statement from the Association of Combattant Clerics, headed by reformist ex-president Mohammad Khatami.

Moderate President Hassan Rouhani has tried to play down the unrest, which began over economic woes but quickly turned against the regime as a whole with chants of 'Death to the dictator'.

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In a statement Monday he called them 'nothing' and vowed Iranians would deal with 'this minority who... insult the sanctities and values of the revolution'.

Pro-regime rallies were held across several towns and cities - reflecting continued support among a large conservative section of society.

The head of Tehran's revolutionary court, Moussa Ghazanfarabadi, warned that as violence grows punishments for demonstrators would get 'heavier'.

'We no longer consider them as protesters demanding rights, but as people targeting the regime,' he told the conservative Tasnim news agency.

Rouhani came to power in 2013 promising to mend the economy and ease social tensions, but high living costs and a 12 percent unemployment rate have left many feeling that progress is too slow.

The young are most affected, with as many as 40 percent out of work according to analysts, and rural areas particularly hard-hit.

'People have had enough, especially the young people. They have nothing to be happy about,' Sarita Mohammadi, a 35-year-old teacher in Tehran, told AFP.

'The situation is far worse in provinces. Agriculture has been destroyed. I know many who have left the north of the country to come to Tehran to work,' she added.

Rouhani acknowledged there was 'no problem bigger than unemployment' in a speech on Sunday, and also vowed a more balanced media and more transparency.

Turkey on Tuesday expressed concern about the unrest in a statement that called for 'common sense' to 'prevail to prevent any escalation'.

The European Union on Monday pushed Iran to guarantee the right to protest.

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In 2009, authorities ruthlessly put down protests against the re-election of hardliner Mahmoud Ahmadinejad. At least 36 people were killed in 2009, according to an official toll, while the opposition says 72 died.