Iranian authorities cut off phone service and internet access Thursday in the capital and in several parts of the country as mass protests and chanting against the government continue, with dozens of people killed in the demonstrations and thousands arrested. Multiple sources in Tehran told CBS News the internet was down in the capital.

The NetBlocks monitoring organization said Thursday evening local time in Iran that its live data showed Iran was "now in the midst of a nationwide internet blackout; the incident follows a series of escalating digital censorship measures targeting protests across the country and hinders the public's right to communicate at a critical moment."

Attempts to dial landlines and mobile phones from Dubai to Iran could not be connected, the Associated Press reported.   

Security forces confronted protesters in several cities and towns, firing tear gas. One CBS News source in the capital said there were "huge crowds out across Tehran. Unprecedented," and confirmed that the internet was down for most people in the city. He said some people, with more robust, more reliable business accounts could still get online. Not long after, that source became unreachable, suggesting the blackout had widened even further.

There were reports on social media, largely by anti-regime activists, that web service was also down or severely restricted in the cities of Esfahan, Lodegan, Abdanan, and parts of Shiraz.

The web outages came as Iranians began chanting out of their windows against the regime, following a call by exiled Iranian Crown Prince Reza Pahlavi, the son of the former U.S.-backed shah, to make their voices heard at 8 p.m. local time (noon Eastern). Analysts and insiders told CBS News the scale of the response to Pahalvi's call could determine whether the deadly, 12-day-old protests fizzle out as previous rounds of unrest have, or grow into a major challenge to the government, and provoke a possible wider crackdown.

"All of the huge crowds in my neighborhood are pro-Pahlavi and from several areas my sources report the same — pro-Pahlavi crowds are prevailing, undeniably," the source in Tehran told CBS News, calling it "monarchists responding to Reza."

So far, the unrest has left at least 42 people dead, including at least four members of the security services, and seen more than 2,260 others detained, according to the U.S.-based Human Rights Activists News Agency.

President Trump told conservative radio host Hugh Hewitt Thursday that his administration is monitoring the protests in Iran. He threatened to take severe action if authorities kill protesters. 

"I have let them know that if they start killing people, which they tend to do during their riots — they have lots of riots — if they do it, we are going to hit them very hard," Mr. Trump said. 

Speaking to reporters Thursday at the White House, Vice President JD Vance said the U.S. stands by anybody in Iran engaged in peaceful protest. Asked if the U.S. would take part in any Israeli strikes on Iran, Vance called on Iran to have real negotiations with the U.S. over their nuclear program.

"I'll let the president speak to what we're going to do in the future," Vance said.

NetBlocks said earlier that its "data show the loss of connectivity on #Iran internet backbone provider TCI in the restive city of Kermanshah as protests spread across the nation in their 12th day; the incident comes amid rising casualties with indications of disruptions in multiple regions."

Iranian authorities regularly restrict or disable internet access when they expect significant protests or other potentially destabilizing events.

President Mahsoud Pezeshkian, seen as a reformer but subordinate to Iran's longtime Supreme Leader Ayatollah Ali Khamenei, intimated ahead of his election in 2024 that he would free up the internet and make more websites accessible. It remains tightly restricted, however. Social media sites such as TikTok, Facebook and X are officially banned, as is access to U.S. and European news sites, including CBS News.

Many young, tech-savvy Iranians have become adept at getting around the restrictions, but it's a cumbersome process, and when the regime slows down internet speeds at politically sensitive times, the whole system can become unusable.