TEHRAN, Iran — Iran blocked access to Facebook on Saturday in what opposition candidates said was an effort to sabotage their challenges to President Mahmoud Ahmadinejad.

Facebook has become hugely popular in Iran, where young urbanites have become avid users to connect with friends, play online games and share photographs.

Recently, lively discussions had taken place on the social-networking Web site among Iranians who wondered whether voting in the June 12 presidential election meant supporting Iran’s system of clerical rule, or, as some argued, could be used to remove Ahmadinejad.

Ahmadinejad’s three opponents also used the site to spread campaign messages that, until recently, had been ignored by Iran’s government-controlled national broadcaster.

On Sunday afternoon, when Iranian Shiite cleric Mohammad Ali Abtahi wanted to check the latest election discussions on Facebook, he found that it had been blocked by authorities.

“The government wants to prevent all free discussions on the elections,” said Abtahi, a former vice president and adviser to Mehdi Karroubi, one of the challengers, who advocates more civil rights.

“I had more than 3,000 friends,” Abtahi said in a telephone interview. “Those people used the site to discuss freely, something which unfortunately is not possible in our national media.”

“We used Facebook to be in direct contact with the voters,” said Saleh Behesti, 22, an industrial design student who helped organize the Internet campaign of Mir Hossein Mousavi, who has mounted the most serious challenge to Ahmadinejad.

“We had 6,000 people on Facebook sending information on Mousavi’s speeches and meetings out to all their friends,” Behesti said. On Saturday, 20,000 people turned out for a Mousavi campaign rally in a Tehran stadium. “Without Facebook we would have never been able to gather so many people,” Behesti said.

Iran, like many other countries in the Middle East, frequently blocks Web sites, mainly those with sexual content. Decisions on blocking are made by a group known as the Council for Determining Instances of Filtering, consisting of members of the government, judiciary and intelligence.

Many Web pages voicing criticism of Iran’s political leaders are blocked. But major Western media sites, as well as YouTube and the photo-sharing site Flickr, are accessible.