The Public Security Bureau (PSB) is the Chinese police force which deals as the law enforcement, judicial system, and capital punishment system. PSB may be either uniformed or plain clothed.
Generally, the PSB will not bother foreigners unless they are found with opium (and/or its derivatives) or if foreigners blatantly demonstrate pro- Tibetan political opinions. Also, do not attempt to enter/exit Tibet through a closed route or take photos of protests or military sites. You should also be sure to have proper documentation for the areas in which you wish to travel in order to avoid any problems.
For a minor offense, the PSB may fine you or force you to write a guilt confession. It is highly unlikely that the PSB will imprison you for a more serious offense, but it is possible that you will be expelled from China at your own cost.
Elizabeth Tseng