Twitter last night delayed a “critical network upgrade” to ensure its continued operation for the sake of the vital role the online micro-blogging service is playing in Iran. Twitter and its network partners rescheduled the upgrade, which will cause a one-hour shutdown of the service, until 2-3 p.m. Pacific Time today, which is 1:30 a.m. in Tehran.

Yesterday, messages posted on Twitter by Iranians linked to photos and videos, many caught by mobile phone, that wound up on photo-sharing sites and YouTube in defiance of an internet blackout instituted by the government of President Mahmoud Ahmadinejad. The blackout, which kept Iranians from posting developments on Facebook and by email and shut down text-messaging, began shortly after he claimed an early victory in the June 12 presidential election. Twitter account holders sent photos of bloody wounded and announced that a protester was shot dead during clashes in Tehran, even as the BBC and other networks had equipment confiscated. One BBC television crew was arrested and detained briefly. In the meantime, Twitter posts spurred Iranians in the street in defiance of government orders in support of Ahmadinejad’s defeated rival Mir Hossein Mousavi, who appeared at yesterday’s rally for the first time since he was reportedly under house arrest following the election.

Overnight, Iranians on Twitter spread news of growing protests—to Rasht, Isfahan, and other cities. Persiankiwi, which at one point went offline, saying, “we are moving…situation in Tehran tense…can’t explain” today advised protesters to “wear black and avoid clashes” with police in plain-clothes who are otherwise mixing with the crowd. StopAhmadi listed five killed in Tehran yesterday—“Mina Ehterami, Fatemeh Barahati, Kasra Sharafi, Kambiz Shojaei, Mohsen Imani”—and at one point carried news that 100 university students are missing.

Persiankiwi rightly noted the “many stories and rumors” flying in the midst of the chaos—and that Twitter can also be used by government officers and others to spread disinformation. But specific detail from a handful of Twitter users in Tehran (who seem hardly to have slept in the last three days and are more careful to say when reports are “unconfirmed” or “reliable”) is telling a real-time story of defiance in the face of a ruthless Islamic regime.