Tehran, Iran (CNN) -- Iranian protesters are vowing to continue their anti-government demonstrations into Wednesday night, despite violent crackdowns and arrests.



Witnesses in Tehran tell CNN the demonstrators number in the tens of thousands.



The protests are timed to coincide with the 30th anniversary of the hostage crisis at the U.S. Embassy. Iran celebrates the embassy takeover as an official holiday, and tens of thousands showed up to hear anti-American speeches in front of the building that once housed the U.S. diplomatic corps. Many chanted "Death to America."



The anniversary was also an opportunity to reignite the anti-government protests that were sparked in June, following a disputed presidential election.



Thousands of protesters ignored warnings from Iranian authorities to stay home, many chanting "Death to the Dictator" and others saying, "Obama - Either you're with us or with them," referring to the U.S. president.



Riot police and pro-government Basij militia turned out in force to quash anti-government protests.



"We were running from the police in the alleys off of the main streets," said Soheil, an opposition protester, who gave only one name for security reasons. "Strangers were opening up their garages so we could hide until the police went away. I ran into a garage for about 15 minutes. When I went back into the main street, I saw riot police arresting a group of young men, then putting them inside a bank and locking the door.



"Only God knows what will happen to them after that."



iReport: Iranians take to the streets



Soheil, like many other of the demonstrators, took part in the protests this summer. As with previous demonstrations, many images of Wednesday's protest were uploaded to the Internet on the site YouTube.com -- including video of protesters walking over an image of Iran's Supreme Leader Ayatollah Ali Khamenei. Another clip showed a group of women being beaten by what appeared to be Iranian security forces.



Witnesses said many Iranian women took part in Wednesday's demonstration, and were frequently targeted by the Iranian security forces -- which is what also happened during the summer protests.



Video posted on YouTube showed many protesters beaten on the head with batons.



Iranian reformists have chosen key anniversaries to protest the hardline government -- the same anniversaries chosen for government-sanctioned anti-American rallies. Wednesday's was the biggest annual anti-American observance of all. On November 4, 1979, Islamic students stormed the U.S. Embassy. They held 52 Americans hostage for 444 days.



There were also small pockets of protest on Wednesday in the southwest Iran city of Shiraz, according to a resident who witnessed the protests. Demonstrators marched down the main street of Shiraz, yelling "Death to the dictator" and "We will not stand down to you, together we are united," according to Najmeh, who only gave her first name for security reasons.



There was strong police presence on the streets, and the police force seemed much more organized than past occasions, she said.



Earlier in the day, opposition supporters marched defiantly in Tehran's Haft-e-Tir Square, witnesses said. Many held up their hands in V-signs. Others shouted "Allahu Akbar," or "God is great," which has become a slogan of protest. Police blocked all roads leading to the square, creating massive traffic jams.



Witnesses described helmet-clad security personnel beating demonstrators with batons and firing tear gas at Haft-e-Tir Square and in a neighborhood a few kilometers north.



"I had never seen that many riot police and security personnel," a witness told CNN. "They were brought in by the busloads. As soon as crowds gathered somewhere, riot police were there within minutes."



The opposition showed Wednesday that even after five months of government crackdown, people were still willing to take risks.



The disputed June 12 presidential election triggered Iran's most serious political crisis since the Islamic revolution toppled the shah. Led by opposition candidate Mir Houssein Moussavi, thousands of Iranians protested what they believed was a rigged vote that
What followed the June presidential election? Marchers gathered in which city? Where do marchers gather? When did the hostage incident happen? what caused the protests when did this happen what did the protests follow for what did the marchers gather When was the presidential election? Where did the marchers gather? What do the rallies mark? What happened in 1979? What were they protesting?
anti-government protests Tehran in front of the building that once housed the U.S. diplomatic corps. November 4, 1979, a disputed presidential election. Wednesday night, a disputed presidential election. anti-government demonstrations June 12 Tehran key anniversaries Islamic students stormed the U.S. Embassy. presidential election.
question: What followed the June presidential election?, answer: anti-government protests | question: Marchers gathered in which city?, answer: Tehran | question: Where do marchers gather?, answer: in front of the building that once housed the U.S. diplomatic corps. | question: When did the hostage incident happen?, answer: November 4, 1979, | question: what caused the protests, answer: a disputed presidential election. | question: when did this happen, answer: Wednesday night, | question: what did the protests follow, answer: a disputed presidential election. | question: for what did the marchers gather, answer: anti-government demonstrations | question: When was the presidential election?, answer: June 12 | question: Where did the marchers gather?, answer: Tehran | question: What do the rallies mark?, answer: key anniversaries | question: What happened in 1979?, answer: Islamic students stormed the U.S. Embassy. | question: What were they protesting?, answer: presidential election.