One day after Islamic Republic Leader Ayatollah Ali Khamenei denounced  protests and warned reformist leaders against taking to the streets, thousands of people demonstrated in Tehran. They were joined by others in several cities across Iran in ignoring Khamenei's order and voicing their anger against the June 12 presidential election  results. They clashed with Iranian police who used tear gas and water cannons to disperse them.
At least 19 people were killed in Tehran.
Here is a film showing people trying to stop security forces in Tehran:
and a demonstration in Tehran:
Mousavi: I am ready for Martyrdom
Green Vote tweeted that Mahmoud Ahmedinejad's main opponent Mir Hussein Mousavi was among protesters in Inghlab (meaning Revolution) Square, where he announced : “I am ready for martyrdom”.
Green Vote adds  that Mousavi Mir Hossein said told the people: “We have learned from our fathers that the blood of the innocent shall fall upon the guilty.”
Whoshang tweeted  that Tehran is imploding; that protesters are being hurt/killed and that Mousavi says he's ready for martyrdom. He added that several embassies were opened to receive those injured.
Iranriggedelect tweeted  that protests seem to have become distributed across the city after police did everything they could to prevent one large crowd from amassing in one place.
Tehran Bureau reports  that the chanting/slogans were directly aimed at the supreme leader.
Shooresh provides  a minute by minute report on the protest movement in Tehran and has published several photos (one is above).
A video is also available online, showing a protester allegedly being killed at the hands of security forces. Global Voices Online has the link to the footage but will not publish it because of its graphic nature.