On a related note, I've been to a CL game in Istanbul, and here is my experience:
Match: Galatasaray vs. Real Madrid, second leg of the CL quarter final. They lost the first leg 3-0 in Madrid so this game was pretty much useless, but the atmosphere was great nevertheless at Galatasaray's ground.
I was with a friend and we had tickets for the away section. We were not wearing anything indicating that we are away fans (no RM jerseys, scarves or anything). When we got to the fences around the stadium we asked the police for directions to the away section and two policemen insisted on walking with us to it to show us the way (they told us that a couple of Galata fans were attacked in Madrid, hence the protection). After they showed us the gate, we were allowed to either roam outside the stadium or to go in the stadium until kick-off (Kick-off was 2 hours away).
We walked around the stadium, went to the GS store and bought some stuff (scarves, a jersey, a mug etc.). For some reason when we tried to enter the stadium, many fans were forced to leave whatever we bought outside for no reason, some people were even forced to keep their DSLRs with ONE policeman standing outside (Not at a dedicated kiosk or office, with a single policeman who didn't speak English or Spanish). Luckily for us we had some of the items we bought and a DSLR camera in a backpack which they did not search. All what the policeman did was literally write names and numbers of people on the bags (including plastic bags, using ink that fades easily). That was funny they did all that because in the second half, they started letting some ticketless Galatasaray fans into the away end (it was far from full)
After the game ended, many fans found out that their cameras, bags etc. were stolen, and the policeman (and the security forces in general) couldn't do anything about it, which was shocking. My loss was limited to 80 or so pounds, but some people lost items worth 300+ pounds. Finding a cab was not easy although the stadium is in the city and is not as far as the Attaturk stadium. The cab driver that took us back accepted the fee of 40 Liras, only to bulls**t and claim that he said Euros when he dropped us to the hotel. After arguing he took 50 liras.
- Do NOT take any valuables with you to the ground
- Be very careful and suspicious
I am a frequent visitor to Istanbul (Will be my 5th visit in 4 years) and here are some tips:
- Expect sh*t English, even at the airport, hotels and restaurants (unless you're going to fancy establishments)
- Expect to be ripped off by many people (or that they'd try to rip you off at least). Especially cab drivers. For every journey, insist on using the meter.
- Use the metro/underground whenever you can, there are some good apps for it to show you the way
- Enjoy the city! Istanbul is a beautiful city. If you're staying for more than 1 day there check out Sultanahmet
- Walk in groups whenever you can. Police presence is heavy in Taksim and the surrounding area, and it's quite safe there, but in other areas it can feel dodgy. I never had any issues though myself, but I can pass as a Turk so maybe that's why.