(CNN) Iranian Supreme Leader Ayatollah Ali Khamenei on Tuesday blamed Iran's "enemies" for stirring up unrest in the country, as the death toll from days of anti-government protests climbed to 21.

In his first remarks since the demonstrations erupted last Thursday, Khamenei accused the nation's enemies of "joining forces" and blamed them for the protests that have turned violent.

"The enemy is waiting for an opportunity, for a flaw, through which they can enter. Look at these events over the last few days. All those who are against the Islamic Republic -- those who have money, those who have the politics, those who have the weapons, those who have the intelligence -- they have all joined forces in order to create problems for the Islamic Republic and the Islamic Revolution," he said, without naming any particular country.

The US ambassador to the United Nations said any assertions that the protests are designed by Iran's enemies are "complete nonsense," and that the US would ask Security Council members for an emergency session in New York and the Human Rights Council in Geneva to discuss the protests "in the days ahead."

"The people of Iran are crying out for freedom. All freedom-loving people must stand with their cause," Ambassador Nikki Haley said

Key developments

Protesters took to the streets for the sixth straight day.

Nine people were killed on Monday, including seven protesters, state media reports, taking the toll to 21.

450 people have been arrested in the past three days.

President Hassan Rouhani slammed US President Donald Trump for tweeting his support for protesters.

Trump called the Iranian government "brutal and corrupt" in a second tweet Tuesday.

Foreign Minister Javad Zarif says "infiltrators" won't be allowed to sabotage protests.

An Iranian reformist group led by ex-President Mohammad Khatami accused the US of encouraging violence.

Khamenei delivered a statement in Tehran on Tuesday.

Days of violence

The protests have become the biggest challenge to the Iranian government's authority since mass demonstrations in 2009. About 450 people have been arrested over the past three days, according to state media.

The rallies began Thursday over the country's stagnant economy and rising living costs, but they developed into a broader outcry against the government and intensified over the weekend. Many of the protesters are young Iranians tired of the lack of economic opportunity in the country.

Protesters hit the streets for a sixth straight day Tuesday. Small protests -- not as large as previous days -- appeared in pockets of the capital, Tehran, a CNN producer there reported.

Video on social media appeared to show police officers and demonstrators clashing Tuesday evening in the south-central city of Shiraz. In the video, people scatter through streets amid the sounds of yelling and honking car horns. CNN couldn't immediately independently verify the video's authenticity.

Of the nine people killed Monday, seven were protesters. Six of them died in the central city of Qahdarijan when demonstrators stormed a police station and attempted to take guns from authorities, state media reported. The seventh protester was killed in nearby Khomaini Shahr.

A demonstration at the University of Tehran on Saturday

A police officer died in Najafabad after a protester shot at officers with a hunting rifle, according to state media. Three other officers were wounded. A member of the Basij, a pro-government militia, was also killed, in south Tehran.

Video images shared on social media from the central city of Tuyserkan on Sunday showed protesters throwing chairs, tables and other objects at riot police, forcing the outnumbered officers to retreat. Six protesters were shot dead in the unrest there, according to the semi-official Fars news agency.

A man and his young son also died on Sunday when a fire truck hijacked by protesters ran them down on a street in western Iran's Dorud, according to the semi-official ISNA news agency. Twelve people were killed over the weekend.

Social media crackdown

Iranian President Hassan Rouhani on Monday tried to downplay the significance of the protests, which have spread beyond the capital of Tehran to at least 18 cities, claiming that "This is nothing" compared to other outbreaks of unrest.

But authorities have nonetheless responded with mass arrests and by restricting the use of the social media apps Instagram and Telegram, used to organize the rallies.

The US State Department said it is encouraging tech companies to try to keep such sites accessible in Iran.

"The message: We want to encourage the protesters to continue to fight for what's right and to open up Iran," Undersecretary of Public Diplomacy Steve Goldstein said.

An Iranian man shows his phone as certain apps were restricted in Tehran on Tuesday.

There also are now concerns that some protesters could face the death penalty.

Musa Ghazanfarabadi, the head of Iran's Revolutionary Court, said that some protest ringleaders could be charged with "muharabeh" -- taking up arms against the state -- and accused them of being connected with foreign intelligence agencies, the semi-official Tasnim news reports. The crime carries a maximum sentence of death.

US accused of encouraging violence

The Association of Combatant Clerics -- an Iranian reformist group led b Khatami -- acknowledged that Iranians face "livelihood, economic, political, and social problems and difficulties" and said they have the right "to express and even shout their demands legally and through civil protests."

It also called on the government "to listen to the voice of the nation and pave the way for resolving the problems and meeting their rightful demands."

But it also accused the United States of encouraging the violence.

"The grudge-holding and sworn enemies of the nation of Iran, with the US at the top... came to support the rioters and their violent actions," a statement by the group read.

"The bitter events of recent days showed that opportunistic and trouble making elements pursue the dirty agenda of the enemies, by abusing the quiet gatherings and protests of the people, creating riots and insecurity, damaging public property, insulting religious and national values, and even killing innocent people."

The association, with Khatami at the helm, led protests in Iran after disputed election results in 2009.

Foreign Minister Zarif tweeted that Iran would not allow "infiltrators" to sabotage protests.

"Iran's security and stability depend on its own people, who -- unlike the peoples of of Trumps regional 'bffs' -- have the right to vote and to protest," Zarif tweeted. "These hard-earned rights will be protected, and infiltrators will not be allowed to sabotage them through violence and destruction."

Iran's security and stability depend on its own people, who — unlike the peoples of Trumps regional "bffs"—have the right to vote and to protest. These hard-earned rights will be protected, and infiltrators will not be allowed to sabotage them through violence and destruction. — Javad Zarif (@JZarif) January 2, 2018

Rouhani discussed the protests and terrorism in a phone call with French President Emmanuel Macron.

In a statement on the President's website, Rouhani told Macron that a terrorist group based in Paris is "provoking and persuading people to take violent actions in Iran."

"We expect the French government to take its legal responsibility to combat terrorism and violence," the website statement said.

Trump: 'Time for change!'

Trump has repeatedly tweeted his support for the protesters in the past week, criticizing the Iranian government as repressive, "brutal and corrupt."

"The people of Iran are finally acting against the brutal and corrupt Iranian regime. All of the money that President Obama so foolishly gave them went into terrorism and into their 'pockets.' The people have little food, big inflation and no human rights. The US is watching!" he wrote

Rouhani slammed Trump for a similar tweet Monday in which the US leader said the Iranian people were "repressed" and that it was "TIME FOR CHANGE!"

Supreme National Security Council Secretary Ali Shamkhani accused the US, UK and Saudi Arabia of using hashtags and social media campaigns inside Iran to incite riots.

"Based on our analyses, around 27% of the new hashtags against Iran are generated by the Saudi government," Shamkhani said, according to state-run Press TV.

The UK has called on Iran to engage in a "meaningful debate" on the issues raised by protesters, Prime Minister Theresa May's spokesman said.

Trump has not yet made a decision on whether to sign an Iran sanctions waiver, White House press secretary Sarah Sanders said. But she said the administration is keeping "our options open" as far as sanctions against Iran are concerned.

Trump faces a mid-January deadline when he must decide about renewing temporary waivers for US sanctions against Iran.