We have just returned from 9 weeks in Turkey. There were quite a number of demonstrations with some violence in both Istanbul and some other centres such as Izmir. In the first demonstration whilst we were in Istanbul there were police action against the demonstraters, many of whom were injured and hospitalised. There were minor bomb blasts in a number of towns. None of the action interfered with our trip but we were circumspect when we saw police with their riot gear assemble.

I think some people have been confused/afraid about what's been going on in Turkey/Istanbul because of the press. Yes, a lot's going on but they're not riots like you think they are, especially not in the scale of Paris ;P (HaHa, I get to laugh at elitist Europeans every chance I get for things like these)
So, to agree with above posts and to sum up:
For one, as GTI says, we have elections coming up on the 22nd; there have been quite a number of peaceful walks through main arteries of major cities. These were democtatic demonstrations including pregnant women, little old ladies, municipality workers, young boys etc. etc. and were mainly in opposition to the current government (AKP) imposing the(ir) new president of the republic by at the end of its term to call out to the many political parties to unite before the election. As the election is in 2 weeks, there won't be any more of these.
Secondly, there was the 'walk' at May 1st as 'caretta' says; this was the one involving some injury due to the fact that parts of the gathered group got aggressive and would not leave after the walk was over etc.
Thirdly, there have been some walks against terrorism (PKK) as we continue to lose members of the military and civilians. (Remember; in Turkey military duty is obligatory, and every civillian man will be or has been a soldier at one point) These demonstrations outrage were due to the minor bomb blasts 'kalki' describes which happened after 4 years of quiet. Police would have to wear riot gear, as you can understand the population at large is fed-up with people beeing killed by terrorism for over 20 years. I wouldn't want you to be concerned about these any more than you would of a bomb in Europe (namely U.K.) as they're not anywhere you will probably be. Life goes on as usual in Turkey.
And fourthly, as the election is in 2 weeks, there are rallies all over the country, and you'll definitely at one point hear a very loud bus or 'minibus' playing some tune trying to find support for its party. Nothing to concern yourself with.
And to finish all this rambling off, you're coming in September! What're you worrying about!? :)
You will see riot place in full gear attending/observing all those political rallies.They are there and ready just incase something goes wrong so do not feel intimidated that something horrible is actually going on or is expected to happen . This is Just the politicians speaking and their voters cheering.
