From a gamespot post...
-*Incite a Riot: The thief will incite a riot in an enemy city. A city in a riot
will have a little fire icon on it. You cannot incite a riot in a capital. As
long as there's no party with a leader inside the city, a thief always has a
100% chance of inciting a riot, regardless of the city's size or the units
garrisoned in it. Several different things happen when a city is in riot:
-the owner cannot access the city in any way, meaning he cannot build units
in it, cannot rearrange units, cannot grow the city, cannot enter a party
into it, cannot access the city's menu, etc.
-no thieves can act on the city.
-units inside the city cannot regenerate health during the riot like they
normally would.
-any unit inside the city that was initially at full health will take
damage equivalent to 20% of its maximum HP at the end of the riot.
-any unit inside the city that was initially partially damaged will get
healed by about 4% of its maximum health at the end of the riot.
-any unit inside the city that was initially near death will be healed by
five more points than the percentage the city heals by at the end of
the riot. For example, a level 3 city thaat normally heals by 20% will
heal a near-dead unit by 25 points at the end of a riot; a level 5
city that normally heals by 30% will heal a near-dead unit by 35
points at the end of a riot.