The protesters "are angry and they're mobilized. I think it poses a problem for the regime in terms of its ability to respond to the geographic spread of these protests," said Nader Hashemi, director of the Center for Middle East Studies at Denver University.
What began last week as an expression of frustration over Iran's sluggish economy has broadened to include open defiance of the national Islamic leadership itself. The demonstrations are the most serious challenge to the leadership of the Islamic republic since massive street protests over election results in 2009.
Today, however, "you're not just dealing with Tehran. You're dealing with small, rural towns across the country, where the regime's ability to deploy security forces is much less effective," Hashemi said. "I don't think they planned for this type of broad-based revolt."
The latest bloodshed occurred in towns around the central city of Isfahan, about 275 miles south of the capital, Tehran. State television said six of the latest casualties occurred during an attack on a police station in Qahdarijan, which state media said involved protesters trying to steal guns.
An 11-year-old boy and a 20-year-old man were killed in the town of Khomeinishahr, and a member of the Revolutionary Guard Corps was killed in the town of Najafabad, state television also reported.
In his first remarks on the unrest, Ayatollah Ali Khamenei, Iran's supreme leader, posted comments on his official website Tuesday saying the protests were encouraged by the country's "enemies" — often used as shorthand for the United States, their allies and anti-government Iranian exiles.
"In recent days, enemies of Iran used different tools including cash, weapons, politics and intelligence apparatus to create troubles for the Islamic Republic," said the statement from Khamenei.
He made no comment on how security forces should confront the demonstrations, saying only that he will address the nation "when the time is right." But other top officials have called for a harsher security response. Khamenei's claim of outside involvement in the protests suggested that he viewed the events as more than a domestic upheaval, and his assertion could presage much tougher measures.
In Washington, the Trump administration accused Iranian authorities of blocking electronic communications used by demonstrators and signaled that it might press for new sanctions against Iran because of human rights abuses.
Particularly worrisome for Iran's leaders is the spread of the protests into provincial areas, traditionally conservative strongholds not often drawn into the political activism led by groups in Tehran and other cities.
By contrast, the mass demonstrations in 2009 were centered in Tehran and spearheaded by middle-class and wealthy supporters of reformist political candidates. Those protesters were outraged by the outcome of the national election, which they said had been stolen by hard-liner Mahmoud Ahmadinejad.
The unrest that began Thursday in the northern city of Mashhad was triggered by sharp price increases and other economic troubles. These pressures at home come as Iran has been pursuing an assertive policy abroad, including armed campaigns in Syria and Iraq.
"I've been out of work for months," Rahim Guravand, a 34-year-old construction worker in Tehran, told the Associated Press. "Who is accountable for this? The government should stop spending money on unnecessary things in Syria, Iraq and other places and allocate it for creating jobs here."
He warned that economic difficulties were making people desperate but not necessarily violent.
"If authorities do not fight protesters, then they will have peaceful protests," Guravand said.
Nasrollah Mohammadi, a mechanic near Tehran's Enghelab Square, the site of many past protests, told the Associated Press that he shares the demonstrators' concerns.
"They are right. Corruption is high and opportunities are given to their own friends," Mohammadi said, referring to government officials. "I have two sons, 27 and 30, at home without jobs years after graduation."
The causes of Iran's economic doldrums are many — top-heavy bureaucracy, mismanagement and other problems — but some of the current complaints over consumer complaints go back to the conservative government of Ahmadinejad, who was once a hero to many of those now in the streets.
In 2010, the government began a phased withdrawal of price subsidies for fuel and some food staples such as bread, flour and rice. The move was needed to offset the economic squeeze from international sanctions, but it was deeply unpopular among Iranians long accustomed to price controls on some foods and among the lowest fuel prices in the world. President Hassan Rouhani, a relative moderate, cut fuel subsidies after taking office in 2013.
As violence has intensified, there is little apparent evidence of cracks in the regime, including the ruling network of clerics and the powerful security apparatus.
A semiofficial news agency reported Tuesday that 450 people have been arrested in Tehran since Saturday. The Iranian Labor News Agency cited Ali Asghar Nasserbakht, a security deputy governor of Tehran, in its report on the number of arrests in Tehran. No figure has yet been offered for other cities.
The head of Tehran's Revolutionary Court warned that arrested protesters could face the death penalty when they come to trial, the Associated Press reported.
Iran's semiofficial Tasnim news agency quoted Mousa Ghazanfarabadi as saying: "Obviously one of their charges can be Moharebeh," or waging war against God, which is a capital offense in Iran.
Brian Murphy in Washington contributed to this report.