ISTANBUL — Iran’s most powerful security force threatened ­anti-government demonstrators Monday with “decisive action” after days of protests over abrupt fuel-price increases, signaling a potentially brutal clampdown as officials scrambled to quash the unrest.

The Revolutionary Guard Corps, an elite military force, said it would take “decisive and revolutionary action against any continued moves to disturb the people’s peace and security,” according to a statement carried by state media.

The warning came as the government continued to restrict Internet access, saying it was unclear when the controls would be lifted.

“The Internet will be reconnected in some provinces where there are no security issues,” government spokesman Ali Rabiei said Monday, local media reported. He said he thought that the protests — which erupted across Iran — had subsided in the majority of the country.

State media published images Monday of shattered storefronts, charred vehicles and ransacked buildings allegedly targeted by demonstrators in major cities, including the capital, Tehran.

The size and scale of the unrest remained unclear Monday because of the Internet shutdown. In the central city of Isfahan, residents said, a sense of quiet prevailed after a night of intense clashes with police. In Mashhad, in the northeast, the streets were similarly deserted. In some cities, the government ordered schools and public transportation closed.

The New York-based Center for Human Rights in Iran said Monday that at least 12 people had been killed, citing local media reports. Officials confirmed Sunday that two people — including one protester and a police officer — were killed.

The protests first flared Friday just hours after the government announced that it was raising the price of fuel for consumers. Iran, which has the world’s fourth-largest crude oil reserve, also maintains some of its most heavily subsidized gas prices.

An economic council chaired by President Hassan Rouhani, raised the cost of gasoline by 50 percent, from 10,000 to 15,000 rials, or 13 cents per liter (15 cents per gallon).

At the new price, consumers are allowed to buy up to 60 liters of gas, after which they will be charged double the amount per liter. Before the increase, consumers could purchase as many as 250 liters at the subsidized price.

The surprise measure was designed to raise funds for cash handouts to about 60 million lower-income Iranians. And on Sunday, the country’s supreme leader, Ayatollah Ali Khamenei, declared his support for the move.

But ordinary Iranians, fed up with runaway inflation and a weak currency, balked at the decision. In recent years, Iran’s economy has faltered under U.S. sanctions, and similar moves to cut subsidies prompted a wave of protests two years ago.

The United States has embarked on a campaign of “maximum pressure” against Iran, a strategy it hopes will compel Tehran to abandon its nuclear energy and ballistic missile programs. A key pillar of the campaign includes a near-total embargo on the Iranian economy, including halting oil exports, blocking financial transactions and barring companies from investing in its aviation, automotive and shipping industries.

On Monday, judiciary chief Ebrahim Raisi — a key member of the economic council that announced the decree — acknowledged the protesters’ concerns and conceded that the government had failed to properly communicate the reasoning behind the decision.

“It was necessary to inform the public beforehand,” he said.

The government would begin distributing cash handouts funded by the price increases to about 20 million Iranians beginning Tuesday, state media reported.