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LONDON — Iran's most significant protests in almost a decade may have calmed, but anger that fueled the nationwide demonstrations lingers and could erupt again at any time, according to experts.

Suzanne Maloney, deputy director of the Foreign Policy program at the Brookings Institution, called the recent rallies "one of the most serious crises Iran has faced in the past 25 years."

What made this mass display of dissent — which resulted in at least 21 deaths and more than 3,700 arrests — so significant was that it appeared to be leaderless and spread without the impetus of any particular opposition group.

"We now see that Iranians are willing to take profound risks to challenge the regime directly in a way we have not seen in years," Maloney said.

Late last month, hundreds of people took to the streets in the country's second city of Mashhad. They railed against skyrocketing prices and took the rare step of chanting "death" to their leaders, President Hassan Rouhani and Ayatollah Ali Khamenei, Iran's supreme leader.

Although some have claimed the initial protests may have been initiated by hardline enemies of Rouhani, who is considered a moderate, their proliferation to dozens of cities including Tehran appeared entirely spontaneous.

Iranian Supreme Leader Ayatolllah Ali Khamenei greets the crowd during a meeting in Tehran on Tuesday. Iran's Office of the Supreme Leader via EPA

Many of the activists appeared to be young, disenfranchised working-class Iranians, giving the uprising a wholly different tenor to the Green Movement of 2009, which saw millions take to the streets and was driven by the middle classes.

Alongside high youth unemployment and inflation, many ordinary Iranians feel that the economic benefits they were promised following the U.S.-backed nuclear deal of 2015 failed to trickle down into their pockets.

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Some 42,000 people took part in the nationwide protests, according to Iranian officials.

"The Iranian government dodged a bullet, thus far. Now it's a race against time for the government," said Holly Dagres, a Middle East analyst and curator of The Iranist newsletter. "Very real grievances remain and they will need to be addressed in real-world terms. If they don't, expect protests to flare up again and who knows what that will lead to."

Both Rouhani and Khamenei have acknowledged the protesters' right to be heard. The latter, the figurehead of the country's conservative clerical elite, even conceded: "We must listen, we must hear. We must provide answers within our means," according to Reuters.

But the supreme leader also blamed the U.S. and its arch regional enemy, Saudi Arabia, for helping foment the protests' most violent aspects. And many Western analysts say that the regime — both moderates and hardliners — need to back up their words if they are to appease their simmering population in the long term.

"Without addressing the main drivers of these protests, the Iranian leaders are buying time until the next one," said Ali Vaez, director of the Iran Project at the International Crisis Group, a non-profit organization working to prevent wars.

Siavush Randjbar-Daemi, a lecturer in Iranian history at Britain's University of Manchester, predicted "a very, very testing next few months" for the regime.

"On the one hand it has succeeded in preventing these protests getting out of hand and contained them to a great extend and they did not achieve critical mass in terms of numbers," Randjbar-Daemi said. "However, I do not think the feeling of dissent toward the regime and level of disgust for the political and economic situation has gone away."

The economic and political flavor of the demonstrations suggested discontent with both the country's finances and political system. Reforming either won't be easy.

Iran is part democracy, part theocratic regime. Citizens elect their president and lawmakers, but these representatives are all heavily vetted by appointed, powerful religious institutions.

The country is guilty of suppressing freedom of the press and peaceful protests, as well as cracking down on dissent and overseeing unfair prosecutions and excessive use of the death penalty, according to independent watchdogs.

Students clash with riot police near the University of Tehran on Dec. 30. EPA

On top of this, the country's economy — which Randjbar-Daemi described as "very complicated" — is in a mess, with unemployment and inflation both dangerously high.

Solving any of these issues would likely require some degree of cooperation between Iran's hardliners and moderates like Rouhani.

The sitting president was elected in 2013 on the promise of delivering prosperity. Yet today inflation has crept up to 10 percent prices food such as eggs and poultry have spiked dramatically, according to The Associated Press.

Unemployment is at 12.7 percent, according to the most recent World Bank figures, and most analysts say the figure is far higher among young people.

"These issues — economy, corruption, and repression — are a threat to Iran's stability," according to Dagres, curator of The Iranist newsletter. "If left to fester, they could spark future protests on a larger scale and not just from the working class, but the middle and upper classes as well."

She said not addressing these problems is "almost like playing roulette."