While a pro-government crowd chanted anti-American slogans and burned U.S. flags at the walls of the former embassy compound - still often called the "den of spies" - antigovernment demonstrators were caught in sometimes vicious confrontations at other locations in central Tehran in the first mass protests for six weeks.

Since the 1979 Islamic Revolution, the embassy takeover anniversary has been an important event for rallying regime support, so the scale and boldness of the opposition turnout - after weeks of warnings from security officials that any attempt to gather would be harshly confronted - was seen as a test of opposition strength.

And while the Islamic Republic revitalizes the anti-American pillar of its revolution with a celebration, many of the radical students who took control of the embassy have since become reformist critics.

Ayatollah Hossein Montazeri, Iran's most senior dissident cleric who was at one time the designated successor of Ayatollah Ruhollah Khomenei, the father of the revolution, said in a statement that the embassy takeover and the holding of 52 American hostages for 444 days was a mistake.

"Considering the negative repercussions and the high sensitivity which was created among the American people and which still exists, it was not the right thing to do," he said.

"Some of the revolutionary and committed youth, who were instrumental in that act at the time, now believe that it was a mistake."

U.S. President Barack Obama marked the anniversary of the embassy seizure by saying it had set the U.S. and Iran "on a path of sustained suspicion, mistrust, and confrontation. Iran must choose. We have heard for 30 years what the Iranian government is against; the question, now, is what kind of future is it for?"

Violence erupted last June after a disputed election result that reinstated President Ahmadinejad for a second term. Supporters of challenger Mir Hossein Mousavi called the result a fraud, took their "Green Movement" to the streets by the hundreds of thousands, and were put down during several weeks of unrelenting force that left scores dead.

Widening scope of protests

The protests were once limited in scope to reversing President Mahmoud Ahmadinejad's declared landslide victory in June. But they have expanded in their demands to target the Islamic system led by supreme religious leader Ayatollah Seyyed Ali Khamenei himself, and incorporate a host of complaints from the economy to strict social rules.

On Wednesday, that witness saw trash dumpsters set aflame on two main Tehran avenues, tear gas assaults and arrests. Riot police and ideological basij militiamen would "lead people into side streets [and] start hitting [them] right there and then," he said.

Other witnesses at other flashpoints in the Iranian capital said opposition turnout was lower than the last mass protest six weeks ago, when green-clad demonstrators hijacked official Jerusalem Day ceremonies, and that this time regime enforcers were more violent.