TEHRAN, Iran — The most significant protests in eight years are rocking Iran, with state media reporting Monday that at least 12 demonstrators have been killed.

Hundreds of people have been arrested and activists are taking the rare step of publicly criticizing the country's religious leaders.

The protests started out as local rallies against Iran's economic problems but have since spread in both geography and scope.

Iranians last week took to the streets of Mashhad, the country's second-largest city, in protest at high inflation and the rising prices of everyday goods.

This frustration is hardly new. Many people expected the their financial situation to improve after Iran signed a nuclear deal in 2015 with the U.S. and five other world powers.

The country agreed to limit its uranium enrichment in exchange for sanctions being lifted. The economy has improved — but there is widespread anger that the benefits have not trickled down to ordinary citizens.

The demonstrations spread to Tehran on Saturday, with people chanting anti-government slogans and tearing down political posters.

The next day, officials said that two protesters had been shot dead overnight in the western city of Doroud. Authorities denied they were killed by police, instead blaming foreign agents and terror groups for the deaths.

State TV reported Monday that another 10 people had been killed during clashes.

"Some armed protesters tried to take over some police stations and military bases but faced serious resistance from security forces," state TV reported, according to Reuters.

What started as an isolated economic grievance appears to have morphed into a wider expression of dissatisfaction with the government.

"I think it's far more serious than we anticipated a few days ago," said Sanam Vakil, an associate fellow at London's Chatham House think tank.

How has the government reacted?

Since the Islamic Revolution in 1979, Iran has been a religious conservative society where many aspects of public life, such as freedom of expression, the press and public assembly, are restricted.

On Sunday, Iran's President Hassan Rouhani, who is seen as a reformist, said that while people had the right to demonstrate he condemned "violence and destruction of public property." Read more at NBC News