I think I understand why you aren't allowed to leave, actually you are considered as an illegal resident as you travelled and stayed outside of your allowed stay or residence area for the time limit of a week. They confiscated your passeport, because you tried to flee the reach of authorities (moved around).

The residence registration on your entry card serves you until you leave again, but you will be required to repeat the procedure if some detail or another changes for example, your address, or a visa-renewal. If you're travelling around and staying at hotels, the procedure is taken care of by the hotel itself when you register with them.

the only thing I did do is stay somewhere different than my visa application says. I put where I was actually staying on my entry card (the little yellow/white form they gave me on the plane) - I just changed to a place more convenient about a week before I flew.

You simply forgot to update the information given inside your little yellow/white form and didn't register when you changed your place within 24 hours. Did you?

At least I hope your registrated, when you landed in China and went to the local PSB (Public Security Bureau) office or did it through an hotel staff/travel agency. Otherwise, you are not an illegal resident, but an illegal immigrant and it sucks. :/

Look for “Administrative Regulations of the People’s Republic of China on the Entry and Exit of Foreigners” (Order No.637 of State Council).

Those regulations introduced the new visa system in 2013, but also addressed the illegal entry, stay and employment of foreigners. They have defined the “illegal stay of foreigners” as the following:

• Foreigners who stay and reside in the country beyond the duration of stay stipulated on the visa, stay or residence permit;
• Foreigners who are admitted into China without a visa and have stayed beyond the allowed stay duration without obtaining a stay or residence permit;
• Foreigners who travel and stay outside of the allowed stay or residence area;
• Other illegal stay situations.

It means that you are now considered as an illegal resident. In order to avoid your flight, they preventively confiscated your passeport and won't give it back until they decide what to do with you. It can be very long and difficult, especially because you moved illegaly from one place to another and haven't registered anywhere yet.

Firstly, you need to register somewhere in order to be sure that they know where you are and can track you. Secondly, you have to contact your embassy asap and to ask them to deal with the situation.

Thirdly, go to your nearest visa office and explain the situation to them immediately. Be sure you have a good reason for having left renewal so late. You may find the officials very cooperative if you demonstrate genuine remorse. One week is already long, I hope you haven't been an illegal since you landed in China :/

Penalties for this can be particularly harsh financially. You also run the risk of being held indefinitely in one of China's detention centers for foreigners (with drugs dealers and murderers) until your deportation can be arranged.

Who can't usually get out of China.

• Defendants in criminal cases and suspects who are ratified by the Chinese public security organizations, the People's Procuratorate or the People's Court.
• Those who are informed by the people's court that their civil cases have not been brought to completion.
• Those who have already violated Chinese law; whose behavior has not been dealt with, but is recognized by the legal authorities.
• Those who hold invalid exit credentials, including forged, altered or another's credentials.
• Those who refuse the frontier inspection of China.
• Those who do not exit through the appointed exit port.
• Those who are considered to be carrying contraband goods.
• Those who are forbidden to exit by the Ministry of Public Security and the Ministry of State Security of China.

It can perhaps help everybody : - http://www.chinahighlights.com/travelguide/article/chinese-laws.htm - http://www.travelchinaguide.com/essential/exit.htm