After almost a week of protest, the violent demonstrations rocking the Iranian capital each night are limited in size and confined to less than a square mile. And they remain a leaderless expression of anger.
But what started out as a paltry student demonstration is now loaded with significance for the future of the Islamic Republic.
Unlike the student demonstrations four years ago, say analysts in Tehran, these protests are tapping into an unexpectedly fierce determination by thousands of ordinary Iranians - many of them young, and some families with children in tow - who are frustrated with the slow pace of political change in Iran.
In the past, unelected clerics led by Iran's conservative supreme leader, Ayatollah Sayed Ali Khamenei, were the target of protests. That's true again, with the first-ever public chants calling for the ayatollah's hanging. But now the reform-minded President Mohamed Khatami - whose widespread popularity during six years in office is ebbing, as reforms are systematically blocked - is also a target.
"It's scary talking to these people [the protesters]," says a seasoned political analyst reached by phone in Tehran, who asked not to be named. "There is such a determination in their eyes and their behavior. They are fearless; they are ready for combat. It's like [urban] warfare."
"They say: 'This is just the beginning, we have started it, and we are going all the way to the end,' " the analyst says. "But if you carry on the conversation, they have no idea about what the end should look like.... It is very dangerous."
In recent days, amid the din of supportive honking horns, some protesters have been matching violence meted out by vigilantes loyal to the regime with violence.
Police have issued warrants for a top vigilante, and for some militants who occupied a student dormitory. Scores of protesters have also been arrested, prompting the White House on Saturday to decry "the use of violence against Iranian students" and note that the US "supports their aspirations to live in freedom."
The clerical leadership blames the US - seen officially as the chief foreign meddler since the 1979 Islamic Revolution - for fomenting disorder. Iran's Foreign Ministry accused the US of exaggerating the scope of the protests by calling "a few individuals the voice of the people."
The US comments seem to be uniting hard-liners and reformists. Mehdi Karrubi, speaker of the reformist parliament, said that differences "among the children of the revolution are differences of taste but they are all united against the enemy...."
The US has made no secret of its desire for a new government in Iran. It accuses Iran of backing terrorism, pursuing nuclear weapons, harboring Al Qaeda, and encouraging anti-US forces in Iraq.
But polls have shown that some 90 percent of Iranians themselves want change, and that 70 percent want dramatic change - results that hard-line ideologues say are wildly inaccurate.
"It is going to be a long, hot summer," says Ali Ansari, the British author of "A History of Modern Iran since 1921: The Pahlavis and After." "We're not in a revolutionary situation, but Iran is a country with a long history of social protest, and the situation is incredibly fragile.
"What has been striking in the last year, is the rapidity of the collapse of social popularity of the regime," Mr. Ansari says. "People are no longer saying 'Tinker with the edges, and it will be OK.' People say: 'Let's get rid of them.'"
Another factor conservative clerics are not paying enough attention to, experts say, is that 70 percent of Iran's 65 million population is less than 30 years of age, have little memory of the 1979 revolution, and are the ones who swept President Khatami into power.
Witnesses note that students now make up less than 10 percent of the several thousand demonstrators who have taken to the streets five nights running They also note that Farsi-language broadcasts beamed by pro-monarchist Iranian stations in the United State have helped fan disenchantment.
Leading prayers in Tehran last Friday, Ali Akbar Hashemi Rafsanjani, a former president, warned young Iranians "not to be trapped by the evil television networks that Americans have established."
Analysts see no sign that such conservatives are ready to compromise with the demonstrators, and that the protest may well be crushed in coming days. But the impact appears to widen with every day - with reports Sunday that protests had spread to three smaller cities.
"It's a sign, [like] those small and relatively minor noises that can be heard when a dam is cracking," says Shahriar Rouhani, a professor of physics at Tehran's Azad University. "The engineer knows that this is not a joke ... [and] could result in a major catastrophe."
One sign of change is that Khatami is a special target this time. A majority of reformist deputies last month addressed a letter to Mr. Khamenei, suggesting changes and that he step aside - but hard-liners did not allow it to be published. Scores of reformist newspapers have been shut down, and activists arrested and tried. A final straw for many came when four reformist deputies in recent weeks traveled to several cities outside Tehran to give speeches, and were prevented every time by Hizbullah thugs.
The protests are energizing a political opposition faced with sinking morale in recent months.
"The reformists have been in a dilemma on whether to support the demonstrations," says Sadiq Zibakalam, a political scientist reached by phone at Tehran University. "The overt US support for them puts them in a very difficult situation.... It is debatable whether the US support actually helps the cause, or is counter-productive" because it gives ammunition to hard-liners.
Demonstrators vow to stay in the streets until July 9, the anniversary of the pro-democracy student revolts that were put down four years ago. So far, protesters have not been cowed by the chains and whips used by the hardline militias known as basiji and Hizbullah [soldiers of God] - the shock troops, used to brutally impose order on the streets in the past.
Just 200 yards from the Tehran home of the analyst who requested anonymity, on Friday night "young people using sticks attacked a group of Islamic militants, set fire to their motorbikes and beat them senseless," the analyst says. "Everyone was dancing around the fire."
The seasoned analyst also notes a big change in the perspective of the basijis as a force, which 20 years ago fought against Iraq and its allies.
"You define yourself by your enemies, and those were the superpowers back then," the analyst says. "But now they are fighting young people who put gel in their hair. That's the enemy. So it's demeaning, and not at all elevating for their self-image.
"The only way they can face it now is saying they're fighting these agents of America in our country."
• Material from the wire services was used in this report.