Iran's supreme leader on Tuesday blamed the protests roiling the country on "enemies of Iran" who he said were meddling in its internal affairs, as state television reported that overnight clashes between protesters and security forces killed another nine people.

The demonstrations, the largest in Iran since its disputed 2009 presidential election, have brought six days of unrest across the country and resulted in at least 21 deaths.

The protests began Thursday in Mashhad over Iran's weak economy and a jump in food prices. They have since expanded to several cities, with some protesters chanting against the government and the supreme leader, Ayatollah Ali Khamenei. Hundreds of people have been arrested and a prominent judge on Tuesday warned that some could face the death penalty.

In comments posted to his official website, Khamenei appeared to blame foreign nations for at least exacerbating the unrest gripping Iran.

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President Hassan Rouhani speaks during a cabinet meeting in Tehran on Sunday. Rouhani said people have the right to protest but demonstrations should not make the public 'feel concerned about their lives and security.' (Iranian Presidency Office via Associated Press)

"In the recent days' incidents, enemies of Iran utilized various means — including money, weapons, politics and intelligence apparatuses — to create problems for the Islamic system," he said, in his first public remarks since the demonstrations began.

Khamenei said he would elaborate further in the coming days. Iranian leaders often accuse the United States, Israel and Britain of seeking to overthrow the clerically overseen government.

State TV reported that six people were killed during an attack on a police station in the town of Qahdarijan. It said the clashes were sparked by rioters who tried to steal guns from the police station.

State TV also said an 11-year-old boy and a 20-year-old man were killed in the town of Khomeinishahr, while a member of Iran's paramilitary Revolutionary Guard was killed in the town of Najafabad. It says all three were shot by hunting rifles, which are common in the Iranian countryside.

CORRECTION Italy Iran Protest

A demonstrator shouts slogans outside the Iran Embassy, in Rome, on Tuesday. (Gregorio Borgia/Associated Press)

The towns are all in Iran's central Isfahan province, some 350 kilometres south of Tehran.

It wasn't immediately clear if the Revolutionary Guard member was the same fatality reported late Monday night by Iran's semi-official Mehr news agency. Mehr had said an assailant using a hunting rifle killed a policeman and wounded three others in Najafabad.

It appeared to be the first fatality among Iran's security forces since the protests began.

Government threatens crackdown

President Hassan Rouhani has acknowledged the public's anger over the flagging economy, which has benefited from his signature 2015 nuclear deal with world powers, but not in a way that has brought immediate gains for most Iranians.

Rouhani and others have warned that the government wouldn't hesitate to crack down on those it considers lawbreakers. None of the protest rallies so far has received prior permission from the Interior Ministry, making them illegal.

In Tehran alone, 450 protesters have been arrested in the last three days, the semi-official ILNA news agency reported Tuesday. ILNA quoted Ali Asghar Nasserbakht, a deputy governor of Tehran, as saying security forces arrested 200 protesters Saturday, 150 Sunday and 100 Monday. So far, authorities have not released a nationwide figure for arrests.

The head of Tehran's Revolutionary Court reportedly warned Tuesday that arrested protesters could potentially face the death penalty when they are put on trial.

"Obviously one of their charges can be Moharebeh," or waging war against God, Iran's semi-official Tasnim news agency quoted Mousa Ghazanfarabadi as saying. Moharebeh is punishable by death in Iran. He was also quoted as saying some protesters will come to trial soon on charges of acting against national security and damaging public property.

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Demonstrators gather in Tehran on Saturday to protest Iran's weak economy. (Associated Press)

Iran's Revolutionary Court handles cases involving alleged attempts to overthrow the government.

The protests began over Iran's economy, which has improved since the nuclear deal that saw Iran agree to limit its enrichment of uranium in exchange for the end of some international sanctions. Tehran now sells its oil on the global market and has signed deals to purchase tens of billions of dollars' worth of Western aircraft.

That improvement has not reached the average Iranian, however. Unemployment remains high, and official inflation has crept up to 10 per cent again. A recent increase in egg and poultry prices by as much as 40 per cent, which the government has blamed on a cull over avian flu fears, appears to have been the spark for the protests.

'The UN must speak,' says U.S.

The United States is calling for the UN Security Council and Human Rights Council to hold emergency meetings on Iran. The U.S. is hailing the demonstrations as courageous.

American UN Ambassador Nikki Haley said Tuesday that "the UN must speak" on the issue. She said the U.S. will be calling for emergency sessions in the coming days.

"Nowhere is the urgency of peace, security and freedom being tested more than in Iran," Haley said, adding U.S. officials "applaud the tremendous courage of the Iranian people."

U.S. President Donald Trump on Tuesday wrote on Twitter that "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,"' Trump wrote, apparently referring to the nuclear deal reached under his predecessor. "The people have little food, big inflation and no human rights. The U.S. is watching!"

It is unclear what effect Trump's string of tweets is having on the protests. Some have shared them online, but many in Iran distrust him because he has refused to re-certify the 2015 nuclear deal and his travel bans have blocked Iranians from getting U.S. visas.