The demonstrations are the largest seen in Iran since the country's disputed presidential election in 2009. The Associated Press says at least 21 people have died in the unrest.
The protests began Thursday in Mashhad over Iran's weak economy and an increase in food prices. They have since expanded to other cities, with some protesters denouncing the government and Iran's top leader, Ayatollah Ali Khamenei.
这次抗议于上周四在马什哈德市爆发，矛头对准了伊朗的经济疲软和食品价格上涨。抗议活动已经扩大到了其它城市，同时一些抗议者开始声讨政府和伊朗最高领导人阿亚图拉·阿里·哈梅内伊（Ayatollah Ali Khamenei）。
Hundreds of people have been arrested. A well-known judge warned on Tuesday that some could face the death penalty.
On Twitter, Trump wrote that "all of the money" that former President Barack Obama, in his words, "so foolishly gave" to Iran as part of a 2015 nuclear agreement "went into terrorism and into their 'pockets.' The people have little food, big inflation and no human rights." He added, "The U.S. is watching!"
After Trump's attack, Iran's foreign ministry said Trump was "wasting his time sending useless and insulting tweets regarding other countries." It urged Trump to attend to domestic issues affecting the United States, such as, in its words, "daily killings of dozens of people...and the existence of millions of homeless and hungry people."
Earlier Tuesday, Khamenei made his first public comments since the demonstrations began.
"In recent days," he said, "enemies of Iran used different tools... to create troubles for the Islamic Republic."
Iranian state television broadcast his statement.
Khamenei said he would talk more about the protests in the coming days.
Iranian President Hassan Rouhani said on Monday that security forces would "respond to rioters and lawbreakers."
Deputy Interior Minister Hossein Zolfaghari said 90 percent of those who have been detained are under 25 years old. Many young people are unhappy about a lack of social freedoms and worsening economic conditions in the country.
Turkey's Foreign Ministry said Tuesday it hopes foreign involvement will be avoided in Iran, and that it is concerned about the reported deaths. The ministry said violence should be avoided.
Israeli Prime Minister Benjamin Netanyahu called the protesters "brave" and "heroic."
British Foreign Secretary Boris Johnson called on all those involved to avoid violence.
Johnson also said, "We believe that there should be meaningful debate about the legitimate and important issues the protesters are raising and we look to the Iranian authorities to permit this."
Rouhani described the demonstrators as the "minority who chant slogans against the law and the people's wishes."
The Iranian president added, "The people are absolutely free in expressing their criticisms and even protests. But criticism is different to violence and destroying public property."
Hussein Banai teaches at Indiana University in the United States. He told VOA that many earlier protests in Iran have been led by academics and activists. This time, he says, working classes are leading the demonstrations. And they are doing so in a less organized way.
侯赛因·巴奈（Hussein Banai）在美国印第安纳大学（Indiana University）任教。 他对美国之音表示，伊朗过去很多抗议活动都是由学者和活动家领导的。而这一次，工人阶级开始领导示威活动，组织比较松散。
The Trump administration says it is "very concerned" about Iran blocking its people from communicating through social media in an effort to weaken the protests.
A State Department official said Tuesday that the U.S. is urging Iranians to use VPNs, or virtual private networks, to get around the government's restrictions.
Iran restricted use of Telegram and Instagram on Sunday. State media said the moves were meant to keep peace. Iranians had been using the apps to communicate about the street demonstrations.
I'm Ashley Thompson.