CAIRO — An opposition activist spreads word of an upcoming protest in the streets of Tehran. Another posts pictures of clashes between demonstrators and police.

As Iran's government cracks down on traditional media after the country's disputed presidential election, tech-savvy Iranians have turned to the microblogging site Twitter.

Its use to organize and send pictures and messages to the outside world — in real time as events unfolded — was a powerful example of how such tools can overcome government attempts at censorship.

Iranians must outsmart government blocking to use Twitter, on which users post messages limited to 140 characters called "tweets."

Twitter and other social-networking sites remained blocked yesterday in Iran. Users must go to other sites that post tweets for them and allow them to read tweets from others.

Twitter had planned to go down for 90 minutes yesterday for maintenance, but rescheduled it, citing "the role Twitter is currently playing as an important communication tool in Iran," according to a notice posted on its site. The maintenance will instead happen a day later, when it is 1:30 a.m. in Iran.