not sure of this (I mean the passport). Probably true that you should always have a form of ID with you, like a driving license or an ID card. As suggested by others, an internet search should tell what the rules are in Turkey.

thinking twice after reading other posts:
There are 2 situations when police may ask you to provide a form of ID. If you commit an offense or in case of random check to verify if your presence on national territory is legal. In the 1st case, at least in France, you are not fined on the spot and thus need to provide ID to send you the fine. In the 2nd case, I guess that a passport may be needed, depending on the country you are visiting and your nationality. Still in France, random checks have become a very sensitive issue and must abide to strict rules (for example, OK near a border, or take the opportunity of even a small offense, like not punching your metro ticket, to ask for your passport). As stated by other fodorites, if you look like a bona fide tourist and not an illegal immigrant, police is likely to be lenient in many countries if you don't show a passport.

I regularly cross France Spain border by bus and such checks are frequent. Even on local buses in the area.
And it is the same experience, that is police barely checking the passports.
the reason is simple: they are after illegal immigrants but they cannot discriminate.So, they need to act as if they were checking everybody but it is far easier to detect an immigrant by the way he looks like.

Unless you're planning to rent a car or write traveler checks or do some other bank-related work, there's no need for your original documents and it's probably a good idea to keep it safely stored in your hotel room.

If you want to visit the entry hall of the Zurich police station which used to be an orphanage, you must surrender your passport, not a copy, at the desk.

There are better ways of getting valid and accurate information than by asking a travel form. Check government travel advisory sites.

This is a quote from an official UK gov website for Turkey specifically:

Personal ID

"It is illegal not to carry some form of photographic ID in Turkey. Always carry your passport or residence permit. In some busy areas, especially Istanbul, the authorities may stop people for ID checks. There are also several police checkpoints on main roads across Turkey. Cooperate with officials conducting checks."

US driver's licenses might not be accepted as a valid photo ID in Italy, because there is no national standard in the US, and the police can't be expected to recognise if yours is genuine. A marshall of the carabinieri told me that this is one reason why it's important for Americans to get an IDP if they plan to drive in Italy.

For US citizens an alternative to a passport while out & about might be a passport card. When I renewed my passport last year I opted to also get a passport card for, I think, an additional $35. I have no idea what I’ll use it for but figured if I didn’t get it I’d soon find out. I realize it’s of no use to the OP but for others here it might be useful, myself included. It would have the effect of a federal id rather than a just a state driver’s license for times when one’s passport book is not necessarily required. Just a thought.

I was thinking of getting one too, for the same reason, but for some purposes it may not suffice. I read somewhere that the police might require a real passport, which shows your entry date. Except that lately they never stamp them on entry. When they scan it, the entry date gets entered into an EU database, so they can look it up. So couldn't they access the database with the information on the passport card?

I just returned from Europe on July 2. We came in through Spain and out from Italy. We did get stamps both coming and going. (Without asking for stamps) But when I went to Argentina in December, I didn't get any stamps. I asked the customs agent and she said they have stopped doing stamps. I have no doubt it's all in a computer, and I'm a little surprised they're still doing it in Europe. I think stamping is likely to stop in the near future.

I suppose it might depend on which government agency is asking as I’d be surprised if they all have access to immigration records. And presumably that would vary country to country. But, for those who really don’t want to carry their passport book, I’d think the passport card would be better than, say, a driver’s license as it would be electronically connected to the record of one’s movement in & out of a country. A new mystery to solve.