Tens of thousands of supporters of Mir Hossein Mousavi, the defeated main challenger in the disputed Iranian presidential election, rallied in central Tehran, video on Iranian state television showed, after the biggest protest in 30 years led to as many as 15 deaths.

Since the election, reformist Web sites, as well as Twitter and Facebook, have been cut off in Iran, although Iranians are evading the controls via proxy servers, which can disguise a user’s location to be able to organize themselves and send information.

“We realize Twitter is playing an important role in the events taking place in Iran,” San Francisco-based company spokeswoman Jenna Sampson said in an e-mailed statement. “We’ve seen many instances of people using Twitter to communicate when there was no other way.”

Iran has one of the world’s youngest populations, with as many as 70 percent under the age of 30, born after the Islamic Revolution.

The ability to communicate translates into a desire for political freedom, said Jennifer Windsor, executive director of democracy-promotion group Freedom House in Washington.