This photo, obtained by the Associated Press outside Iran shows anti-government protestors chanting slogans in Tehran on the sidelines of state-sanctioned rallies to mark the 30th anniversary of the U.S. Embassy takeover on Nov. 4, 2009 (AP Photo)Iranian security forces wielding batons and firing tear gas clashed with crowds in Tehran on Wednesday in an effort to break up anti-government protests organized on the same day as state-sanctioned rallies celebrating the 30th anniversary of the U.S. Embassy takeover.

The rally of opposition demonstrators was the first show of force since a mid-September protest that coincided with a state-sponsored rally against Israel.

The protestors again turned out to display opposition to the government of President Mahmoud Ahmadinejad, whose win in a disputed June election sparked the biggest political turmoil since the 1979 Islamic Revolution.

Witnesses said paramilitary units from Iran's Revolutionary Guard swept through the opposition counter-demonstration in Tehran's city centre, clubbing, kicking and slapping protestors. There were no immediate reports on injuries or arrests in the clashes.

However, reports from Tehran are limited because foreign journalists are either barred from Iran or severely limited in their movements. As a result, news coming from Iran is trickling out through first-person accounts, videos posted on YouTube or other online social media tools. Opposition websites have been calling for a large turnout.

Protest follows warning from Ayatollah

Many marchers Wednesday wore green scarves or wristbands as symbols of the campaign of Mir Hossein Mousavi, who claims President Mahmoud Ahmadinejad stole the election through fraud. Mousavi and his allies, including former President Mohammad Khatami, appeared to encourage opposition protesters to return to the streets.

Waving flags, Iranian school girls attend an annual demonstration in front of the former U.S. Embassy in Tehran on Nov. 4, 2009, in a ceremony commemorating the 30th anniversary of the seizure of the U.S. Embassy by militant students. (Vahid Salemi/Associated Press)

The anti-government protest took place about a kilometre away from an annual anti-American gathering outside the former U.S. Embassy to mark the day it was seized in 1979, an event that triggered a hostage crisis that endured for 444 days.

U.S. President Barack Obama acknowledged the anniversary of the hostage crisis, saying in a statement that it had "helped set the United States and Iran on a path of sustained suspicion, mistrust and confrontation."

Obama said he wanted "to move beyond this past, and seeks a relationship with the Islamic Republic of Iran based upon mutual interests and mutual respect."

White House spokesman Robert Gibbs said Wednesday that Washington was following reports out of Tehran and hoped the violence would not spread.

French Foreign Minister Bernard Kouchner also expressed concern, warning the situation was "dangerous."

The protests come after Iran's supreme leader, Ayatollah Ali Khamenei warned in a speech over the weekend that challenging the results of the June election was "the greatest crime."

Earlier in September Khamenei had warned the country's reformist opposition on Friday it would face a "harsh response" for confronting the Islamic establishment.

Opposition members say at least 72 protestors were killed during the protests that followed the election, while Iranian officials said 36 people died.

With files from The Associated Press