I can't really make a synthesis of what Greece has gone (is still going?) through today, focused as I have been into more nearly protests like the Basque general strike and the repeated protests in Madrid. But I will try to briefly mention what I could gather from various sources...
Following them and complementing with scattered twitter
reports, it seems that, since the beginning of the journey, police attacked pickets in the capital (and surely also elsewhere). Several facilities were occupied by workers.
At 13:00 there was a vast demonstration in Athens of 200,000 people or more, marching to the Parliament behind the banner of the municipal workers' union. March whose photos (retouched to prevent repression, I guess) are available at Athens Indymedia
(again online). Some examples:
At 13:30 the front of the march arrived to the fence separating them from Parliament, some people attempted to overcome it, information of small groups of Nazis wanting to attack the crowd.
By 14:20 police had already charged, throwing huge amounts of tear gas, initiating yet another day of riots in Athens, one of the oldest cities of Europe.
Greece has strong networks of organized revolutionary citizens, so police were not the only ones to hit. There was organized response as you may have watched on TV.
By 14:35 the situation was a bit calmer again and the demonstration could continue. Around that time, neighborhood assemblies called for a march against police stations in demand of freedom for arrestees, among whom there was at least on journalist arrested for carrying a gas mask (a must-have survival item these days, I'd say).
Around 15:10, Delta police squad attacked Omonoia metro station, throwing stun grenades inside (crazy, if you ask me but that's what they did).
Since 15:25 police attacked Exarcheia, the most socio-politically dynamic neighborhood of Athens, with clashes extending for the next hours and hundreds of arrested people, who are held deprived of lawyer assistance.
There was a demo called for 19:00 at Gardenia Square but OL-FTGS does not have data for later than c. 18:00, when the relative peace of police occupation fell on Exarcheia.
A reasonably good Al Jazeera video-report:
... mentioning not just the protests and clashes but also the massive human tragedy behind them.
I will update if I gather more information (nothing new by 1:00am, sorry).