GEOFF BENNETT: Protests across Iran today continued to grow, despite a nationwide blackout implemented overnight, with sources telling the "News Hour" it is the largest ever cutoff in Iran's history.

AMNA NAWAZ: What started as street marches against crippling inflation in the autocratic state have quickly grown to become one of Iran's largest protest movements in years, as calls for Supreme Leader Ayatollah Khamenei's ouster mount.

Stephanie Sy begins our coverage.

STEPHANIE SY: In Iran, defiance reignited.

Hundreds of thousands marched across the nation, tearing up flags of the Islamic Republic, and chanting "Death to the dictator," Ayatollah Khamenei.

This bloody protester yells: "I am not scared.

I have been dead for 47 years."

That was when Iran's shah, Reza Pahlavi, was deposed in the Islamic revolution, and Ruhollah Khomeini took power.

It's the former monarch's son, the exiled crown prince, who has gone on social media to call for nightly protests in recent days and deal the - - quote -- "final blow to the regime."

Some Iranians are nostalgic for the past, hanging the pre-revolutionary flag on statues of the ruling clerics and waving it in the streets.

The protests began nearly two weeks ago, amid a failing economy and rising prices, and quickly morphed into mass protests across all 31 provinces, with many protesters calling for an end to a government that gives religious leaders ultimate control.

Videos from protests around the country were posted on social media, this one showing protesters hanging nooses off state CCTV cameras as they called for the end of the regime.

Authorities cut off Internet and phone access in Iran on Thursday.

Human rights groups say security forces have killed at least 48 protesters.

Today video surfaced showing lifeless bodies scattered on the streets of Fardis, 25 miles west of the capital; 2,000 protesters have been arrested, and authorities have said they'd be shown no leniency.

More than a dozen security officers have also been killed.

Today, the country's supreme leader blamed the uprising on U.S.

interference.

AYATOLLAH ALI KHAMENEI, Supreme Leader of Iran (through translator): Last night in Tehran and some other places, a bunch of vandals showed up in destroyed buildings belonging to their own country just to please the U.S.

president.

His hand is stained with the blood of Iranians.

STEPHANIE SY: But President Trump repeated a warning to Iran today.

DONALD TRUMP, President of the United States: I just hope the protesters in Iran are going to be safe, because that's a very dangerous place right now.

And, again, I tell the Iranian leaders, you better not start shooting, because we will start shooting too.

STEPHANIE SY: Iranian Foreign Minister Abbas Araghchi has dismissed Trump's threats.

ABBAS ARAGHCHI, Iranian Foreign Minister (through translator): The possibility of military intervention is very unlikely because it has been a failed experiment in the past.

STEPHANIE SY: But only last June, the United States launched a coordinated attack with Israel on Iran's nuclear enrichment sites.

For the "PBS News Hour," I'm Stephanie Sy.

AMNA NAWAZ: For perspective on these protests in Iran and how the government there is responding, we turn now to Vali Nasr, professor of international affairs and Middle East studies at the Johns Hopkins University School of Advanced International Studies.

His latest book is "Iran's Grand Strategy: A Political History."

Vali, welcome back to the show.

And before we get to the regime and the response, I want to ask you about these protests, the speed with which they grew, the scale to which they have now spread.

What does that say to you about the Iranian people?

VALI NASR, School of Advanced International Studies, Johns Hopkins University: It shows that this underlying anger in Iran is very serious, it's growing.

It has been there for some time.

It's now been aggravated by a worsening economic situation and also a sense of despondency, that nothing's going to change with Iran.

There are no negotiations with the U.S.

There's no sanctions relief.

There's no prospects of real change in Iran.

And all of this is coming to a head.

And I think also the protesters have been emboldened by the support that they're receiving from President Trump and a sense that perhaps the Islamic Republic is more vulnerable this time than it has been in the past.

AMNA NAWAZ: What should we take away from what we see in the way of the regime response so far, as we mentioned, Internet cut off, dozens killed, thousands arrested?

But it's not as brutal a crackdown as we have seen to protests in years past.

Why do you think that is?

VALI NASR: Well, first of all, I think the Iranian regime thinks that, during the June war with Israel, the Iranian public did not rise up.

And that was very important to Iran's survival in that war.

And I think they thought that they had created at least a bond of nationalism with the Iranian public, and they did not want to break that easily by reacting very adversely to the protests.

Secondly, the protests were economic when they began.

And economic protests in the past have not been very threatening to the regime.

It is only when they became much more about toppling the regime, an end to the Islamic Republic, and then they found support from President Trump, that the Islamic Republic decided that the protests needed to be cracked down.

AMNA NAWAZ: What do you make of that threat from President Trump?

I mean, do you believe that the Iranian regime thinks, if they crack down more severely, that could provide a pretext for Americans to intervene?

VALI NASR: I think they take that threat very seriously.

And I think they have assumed all along that domestic stability in Iran is an important signal as to whether Israel or the United States will decide to attack or not.

Secondly, they watched what happened in Venezuela, and they decided that President Trump could cross red lines and take actions that previously nobody assumed that he would.

And it is not beyond a realm of possibility that the United States may attack Iran, may attack, try to eliminate the leadership in Iran in a way that it didn't do during in the past, during the 12-day war.

And I think they think that the direction of the protests may signal to Washington that Iran is weak enough that the regime may fall with a military strike.

So I think they take that very seriously.

And that's what's different this time.

In other words, domestic protests are not in and of themselves threatening.

They are threatening to the regime because they are combined with an external threat coming from the United States.

AMNA NAWAZ: And all factors taken into consideration, what's your assessment of the leadership in Iran right now?

Are they at their weakest point?

Could external intervention topple this regime?

VALI NASR: I think the regime will not be easily toppled.

And the size of the protests have to be much larger for a much longer period of time.

But I think Iran is finding itself in a situation that it has not done before.

Even if these protests go away, the situation before the Islamic Republic is not going to change.

It's still facing a threat of war with the United States and Israel, and its economic situation is only going to get worse.

So I think the regime is not only trying to deal with the protests right now.

It's trying to figure out how it's going to survive.

How might it change in order to improve the situation before it?

And all of this is quite new.

AMNA NAWAZ: And, Vali, in the minute or so we have left, I want to ask you about the exiled Crown Prince Reza Pahlavi, who was the first to call for these protests that we have seen spread across Iran in the last couple of days.

Does the fact that the protest grew as they did say something about his level of support on the ground?

VALI NASR: I think it's very important in rallying the public and giving the protesters a purpose, particularly those protesters who want an end to the Islamic Republic.

They see in him the promise of something new, something different.

And there is nostalgia about the shah's period.

However, he doesn't have a ground game in Iran.

In other words, he doesn't have a political organization, political representation, and he hasn't built a coalition that cuts across various social sectors and social groups in order to be able to take these protests beyond just lashing out against the Islamic Republic towards a vision for the day after.

So I think it's very important at this point in time.

And, however, with the Internet cut off, because he doesn't have political organizations inside Iran, it's very difficult to see how he would be able to influence things going forward.

AMNA NAWAZ: Vali Nasr, always good to speak with you.

Thank you so much for your time and insights.

VALI NASR: Thank you.