Many people were there [at Syntagma square], and they were not leaving. The police made excessive use of gas to evacuate the place, especially given that the great majority were not offering any resistance.
People were walking some distance from the square so that they could breath and staying there or trying to return back to the square from other small streets.
There was no resistance from the people. The incidents of physical violence were unprovoked. When video cameras were present (mostly in the main square) police were more careful. They were much more violent in the surrounding small streets.
Most of the protestors were wearing medical masks and taking Maalox, an antacid medication, in order to alleviate the effects of the chemicals being used by the police, chemicals that induce severe burning of the stomach.
I saw policemen pulling these masks from ladies (50-60 years old) and gas them in the face.
They sprayed the medical room inside the metro-station with gas.
They were using three types of delivery methods: hand-held gas-grenades for short distances inside an enclosed area like the metro-station, gas-grenades fired from guns for long distances inside the crowd and gas-guns spraying directly at people at a distance of 1 to 3 meters or less. The latter was the method used in those cases where people’s masks were removed that I mentioned previously.
After many hours of continuous use, the police must have exhausted the supplies they had with them and at that point they started to throw large stones into the midst of the unprotected crowd. It was really a miracle that none of these stones landed on anyone’s head with lethal consequences, but many were hurt by stones landing elsewhere on their body.
Those victims who were sent to the hospital cannot prove that the stones were thrown by policemen - even in those cases where there were videos catching them in the act.
The official police line is that the stones were thrown by other protestors. It's true that there were some stones thrown at the police as eventually the crowd reacted in anger to the unprovoked attacks. Participants included a wide range of people (not just "anarchists" and not only youth but even older people and entire families). As the police were wearing protective armor the stone throwing presented little danger to them. The protestors however, on the other side, were completely unprotected.
Video of police brutality at Syntagma Square and surroundings 
The police used a new method for reigning in the protests that I had never seen before. They unleashed "attack forces" from around 50 police-motorbikes (with two persons in each one) running wild around the center of the city to a distance of 2-3 kilometers away from the main square and shooting gas grenades whenever they saw more than few people together. Many tourist-groups had the pleasure of being subject to this gassing as well because many of the protestors, myself included, ran to the nearby tourist areas of Plaka and Monastiraki for safety. We were hoping the police would "respect" the foreign tourists. However that was not to be. Whenever they could enclose groups they would fire at them with their gas guns, even inside restaurants, cafeterias, grocery shops or the entrances of apartment buildings.
In some cases they also hit the occupants of these building when the latter had the temerity to tell the police that they are not allowed to go inside a house without a warrant or a public attorney order.
The first attack started around 13:30. By the time I left, exhausted, at around 21:30, the square and surroundings were covered by a fog many meters above the buildings, like the aftermath of the bombardment of a town seen on television. I saw this type of image many times but I never imagined I could see a real "war image" during "peace time".
The subject of police-state and police brutality is well known and wide spread (in the USA especially, as I saw many times on YouTube) and I can write more and more, but I don’t know if it really helps! So I will try to give you the whole picture in a few words.
What we have here is a government following an austerity policy, which doesn’t solve the problem but boosts the debt to such heights that it will never be possible to repay it. And this policy is being pursued solely to protect the banking system, both local and international.
These were mostly European banks that were borrowing money from the European Central Bank at 1-2% interest and lending to us at 4-5% or more. These businesses made profits of several billion per year and the bonuses offered to their “golden boys” were many millions. Now they try to persuade the people of Europe that they will give money to Greece [if the austerity measures are adopted] to cover up their thievery, but all this money will go directly to the banks.
The austerity measures are also imposing on Greece the privatization of every public company or asset (roads, harbors, airports, electricity, water supplies), to be sold at prices less than the profits of one, two or three years of operation. The money that can be raised in this fire-sale is nothing compared to the mountain of debt and the only sure thing is that the state will lose its last few profit making assets. As for the non-profit making assets, no one wants to buy them.
The only source of revenue for the state is heavy taxation (mostly indirect taxes and a capitation tax  ) of people who in the past year saw a significant decrease of their wages or incomes. For example: I have to pay every year about €4500 for social security and now I have to pay €500 for capitation tax in a total income last year of around €7000 and this year around €5000!!!!. In the “good years” I was earning €12000-€14000 per year. If I manage to earn more than €8000 this year I will have to pay even more taxes.
As the citizens movement keeps growing (we are already 35 days into the Syntagma Square protests) the police are attacking whenever the crowd in the square starts to amass more than two or three thousand. The black propaganda against this movement from the main/official mass media is enormous and insidious. Their aim is to keep people frightened in their homes but they don’t care to report that the polls show that 80% of the people are against this policy. Even if their policy was "right" and the people are "wrong", how is it possible for an elected government to use chemical warfare against the majority of its people to pass their policy??? And "punish" every representative who is voting against this policy, respecting the will of the people who vote them, by ostracism!! 
 Video by Dimitris Meletis,
 A capitation tax, sometimes known as a “head tax” is a tax of a portioned, fixed amount per individual in accordance with the census (as opposed to a percentage of income.) The U.S. Constitution forbids a capitation tax though the infamous “poll tax” that was used to disenfranchise African American voters in many Southern States was a form of a capitation tax.
 In Ancient Athens, where the institution of ostracism was invented, a politician who was deemed to be troublesome was often banished from the city for ten years. It is ironic that this tradition is being resurrected by a party claiming adherence to the ideals of contemporary democracy, progress and even socialism.
[Note: The publication of this moving eye witness account of the mass protests at Syntagma Square in Athens and the horrible orgy of police brutality that has erupted there requires no explanation. The PASOK government of George Papandreou forced through a vote in the Greek Parliament last week that will impose draconian austerity measures previously unknown in any advanced industrial country. And it is a government that claims to be “socialist” and “democratic” that is carrying out this policy on behalf of the European banks. The fact that only one PASOK deputy had the courage to vote against the austerity package is a measure of how far Social Democracy has travelled from its post-war role of support for a mild welfare-state. Given the enormity of the betrayals of the traditional working class organizations in Greece, the trade unions, PASOK and their parliamentary representatives, it is natural for a tendency to arise among the protestors to reject all forms of political action in favor of mass protests, strikes and a search for “direct democracy”. We are in complete solidarity with the protestors in Syntagma Square, but their rejection of the political struggle is a serious mistake. History teaches us that the question of political power cannot be avoided. Such were the lessons of the Paris Commune and many other struggles. A. S.]