"There seems little doubt now that something resembling a coup d'état has been attempted in Tehran. The next few days will reveal if it is to succeed. In scenes not witnessed since the mass protests that brought about a revolution that deposed the Shah in 1979, violent demonstrations have broken out in multiple Iranian cities. Crowds chant anti-Ahmedinejad slogans such as "Death to the Dictator". Hundreds of thousands of supporters of the candidate heavily defeated in Friday's presidential election, Mir Hossein Mousavi, have rejected his defeat – claiming foul play. They are joined by key figures from Iran's political elite including the Association of Combatant Clerics who are calling for the result to be annulled. All three defeated candidates, including the conservative Mohsen Rezaee, a former head of the Revolutionary Guards, reject the election results as fraudulent."
"Overnight in what looks like a purge of the opposition up to 100 opposition figures were arrested including reformist politicians such as the brother of ex-President Mohammad Khatami and Mohsen Mirdamadi, a leader of the 1979 hostage crisis; further arrests included journalists such as Ahmad Zeidabadi; other reports suggest Mousavi and fellow reformist candidate Mehdi Karroubi are under house arrest. Although communications across Iran have been severely restricted, it seems that protests continued late into the night with crowds clashing with security forces, the basij militia and vigilante groups."
Update: More news here and here. Oustanding collection of videos and photos here.
Update 2: Iranian government cracks down.
Update 3: Andrew Sullivan at The Atlantic is doing yeoman's service compiling coverage from all over the blogosphere.