Here is a round-up of reports from the ground and pundit analysis (we can't independently confirm eyewitness accounts, so read them with that in mind). Gregory Djerejian has a long, smart post analyzing today's events:

Make no mistake, if a Tiananmen style crackdown ensues, we must condemn it, and loudly. We must reappraise the timing and manner of going forward negotiations. Iran policy will need to be Greenmask re-calibrated on multiple fronts. And I will be even less hopeful for any going forward diplomatic successes, with an increasingly sclerotic, repressive, insecure regime hanging on now well beyond its time. But we should not be, in a fit of ennobled but deeply misguided passion, begin engaging in actions like having President Obama directly contact Moussavi, or deliver a taped message to the Iranian people, and so on. For these actions will be turned on the backs of the people like the young woman massacred in cold blood today in short order. While those here advocating something be done might feel morally superior as they spout such prescriptions from the comforts of far-away New York and Washington, the greater blood spilled should such policy routes be followed will be on their conscience, not those of us counseling against such shallow recklessness masquerading as serious foreign policy. So, to answer this tortured woman’s hauntingly beautiful query which is the subject line of this post, ‘where is this place that we are only screaming to the world with our silence’? It is a horrible place tonight, and the behavior of the ruling Mullahs will ultimately lead to the death of their regime, if not immediately, with the passage of time. But, ironic and hard to accept during this emotional time as it may be, we will hasten that time likely by doing less, rather than more. First, do no harm. The President, I believe, understands this. Hopefully more of his fairer critics will too in the coming days, which will be very emotional ones, I know.

The BBC has several eyewitness accounts:

Today we tried to join the protestors in Azadi and Enghelab square, but every route that we tried was blocked by the police. The plain clothed forces are all Sepah (revolutionary guards), as most of them have the Sepah badge on their clothes. In Sattar Khan I saw with my own eyes two ordinary 40-year-old women being beaten severely with electric batons, for nothing but raising their voice in protest.

The Jpost also got an e-mail from Iran:

Girls are extremely active in all these rallies (a little less in night riots where patches of young men are more visible). They courageously charge anti-riot police, chant slogans in front of them, lead the crowd, etc., but they are equally beaten too. The police seem to have no limit in the use of force. They are disproportionately violent. They don't use fire weapons, but they don't go easy on you with their clubs. They literally beat up protesters to death if they don't get rescued by fellow protesters or somehow break away and run. The level of brutality is exceptional, but it is amazing to see how people stand up to them. I heard from many witnesses that thugs were brought by bus from smaller cities to assist police in the crackdown...

The rigging of the elections and the violent clampdown on peaceful protestors that began today, demonstrates that the uneasy combination of an Islamic state and democracy has failed. By choosing revolution over the remaining vestiges of democracy, the clergy ensured that Iran will no longer serve as a model of mass supported Islamic Revolution. While internally the revolution has been saved, its foreign influence is likely to wane. Nor, as we learned, is it possible to make a peaceful transition from an Islamic to a democratic state, as happened in the aftermath of communism. Instead, Iran is coming to resemble the authoritarian regimes of the region.

One of Nico's readers in Iran:

The chants of death to Khamenei are true...I witnessed peoples fear of the Basij dissapear, an 80 year old chadori woman with rocks in her hands calling for the exacution of khamenei and all Basij...A group of Basij were surrounded and forced in to a building, the front was blocked with garbage and set on fire, They (basij) opened fire on the crowd with what I assume were blanks, the crowed disspersed for a moment the came back with a fury...thats when the molotov cocktails came out. When I moved on the building was on hour later when I passed by again there wasn't much of a building left. There was full blown war...there was a young man who had taken all of a basij's things including their teargas rifle. We were finnaly able to get out on the back of motorcycle...the ride home took 25 minutes,for 15 minutes of it we were passing intermitently though Basij and protesters fires placed to displace the teargas... might I add the 3 hours that we walked through fire we didn't see one shop or car that had been damaged by protesters...however I just recieved word for the one who was kind enough to keep my camera and other belongings that the Basij had gone into her street and destoryed cars...thats all I can get out for now hope some of it may be useful.

Andrew Sprung:

While there appears to have been much disgusting Basij violence, it also seems that the government so far has held back from a full-scale Tiananmen crackdown. Videos seem to show that although protesters were kept from massing in major squares, they did manage to fill streets and stay out en masse. Mousavi, meanwhile, called for a general strike in the event that he is arrested. The fact that he's had the opportunity to make that call indicates something of a standoff.

One of NIAC's contacts:

I was out from 4-10pm. Military and Basijis were everywhere. They wouldn’t let anyone go though. Every time there was a group of us, they would shoot us with water guns and disperse all of us. They wouldn’t let us in to where we were supposed to protest. They had paintball guns which they shot into the crowd and would arrest whoever had a paint mark on them. There was also tear gas everywhere, they would throw it at us and we would throw it back. But it was very dangerous because they all had guns. I saw a body being carried away. People are afraid to go to the hospital to get treatment for fear of punishment. Security and police have been confiscating cameras and arresting those who are taking footage. I saw this young guy taking a video and 5 people attacked him and throughout it all he help his hand up with a peace sign.- then they arrested him. They have also handcuffed students to the Tehran University fence. We talk to some normal police and patrolling cops- they are nice and are trying to help people. But it is the Basij and anti-riot [police] that are ruthless. They have been brought in from out of town. There are also many undercover cops. Also, we don’t watch state media because it takes our hope away. I’m going to go back out, but my cell phone doesn’t work and I don’t know how I will find people.

Another of Nico's contacts in Iran:

You couldn't imagin what I saw tonight, I walked down many streets(Vali asr, keshavars, amir abad, Fatemi, Shademan, Satarkhan, Khosro), and I was injured by tears gas, but the main thing : The big killer group, called "Basij", weared our special military service group -"Sepah"- dresses and they were all armed , I saw by myself one of them had only around 15 years old!!!! and he had the shot order! I saw a girl injured by gon shot (in Amir abad St.)! and there weren't enough ambulances . I walked through Shademan St. they start shooting , a young boy in front of my eyes murdered , and 3 other people were injured , there were also a big fight between people and Basij at Tohid Sq. 7 people was murdered there, I walked from my company to my home , It was taken 4 hours and I couldn't be able to make a video , cause I was in the middle of war!!!!!!!!!!!!!!!!!

Matt Steinglass:

This livestream of events in Tehran is extraordinary and terrifying. It reminds me of Tolstoy’s description in “War and Peace” of the soldier’s-eye view of battle. Nobody has any idea what’s going on, not even the commanders. At the end people retrospectively construct some version of what must have occurred, and create their explanations for why it had to occur the way it did. How terrifying to be the guy with the camera, while across the street they’re firing guns at you.

Tehran Bureau:

I think the government wants neither a massacre nor the marches to continue. Thus they organized their forces  in order to prevent the assembly all together somehow without bloodshed. Today many demonstrators stepped in a well prepared situation and police took advantage of its best units and very well organized command and control system to checkmate them.

The commentary attached to that video of a young woman dying (no idea how reliable this is, but it is going around). From Facebook and YouTube, via the Lede:

Basij shots to death a young woman in Tehran’s Saturday June 20th protests At 19:05 June 20th Place: Karekar Ave., at the corner crossing Khosravi St. and Salehi st. A young woman who was standing aside with her father watching the protests was shot by a basij member hiding on the rooftop of a civilian house. He had clear shot at the girl and could not miss her. However, he aimed straight her heart. I am a doctor, so I rushed to try to save her. But the impact of the gunshot was so fierce that the bullet had blasted inside the victim’s chest, and she died in less than 2 minutes. The protests were going on about 1 kilometers away in the main street and some of the protesting crowd were running from tear gass used among them, towards Salehi St. The film is shot by my friend who was standing beside me. Please let the world know.

One of the lede's readers:

Make special note of that unarmed innocent Girl shot and bleeding from her mouth, nose, eyes, ears…..hundreds of copies just went up on Youtube. The tide of the ‘79 revolution was turned overnight by a similar front-page photo of a Soldier at point blank range shooting an un-armed protester.

Augustus Norton:

If the demonstrations continue for many days, even at the reduced levels seen the day following Khameinei's speech, it is hard to imagine a beneficial outcome for the Leader. His reputation, such as it is, will be further chipped away making him even more vulnerable to criticism from leading clerics. Yet, if an even bloodier crackdown is ordered the regime may insure the unrelenting hostility of many millions of Iranians. Men ofKhameinei's generation will understand the gravity of risk quite well.