I've never thought about it much but are police trained differently for the two situations?
Wikipedia makes the distinction:
Crowd control is the controlling of a crowd, to prevent the outbreak of disorder and prevention of possible riot. Examples are at soccer matches, when a sale of goods has attracted an excess of customers, refugee control, or mass decontamination and mass quarantine situations (disease outbreaks, bioterror attacks, etc.). It calls for gentler tactics than riot control. Materials such as stanchions,<1> crowd control barriers,<2> fences and decals painted on the ground can be used to direct a crowd. Keeping the crowd comfortable and relaxed is also essential, so things like awnings, cooling fans (in hot weather), and entertainment are sometimes used as well. For controlling riots and demonstrations, see riot control.
http://en.wikipedia.org/wiki/Crowd_controlRiot control refers to the measures used by police, military, or other security forces to control, disperse, and arrest civilians who are involved in a riot, demonstration, or protest. Law enforcement officers or soldiers have long used non-lethal weapons such as batons and whips to disperse crowds and detain rioters. Since the 1980s, riot control officers have also used tear gas, pepper spray, rubber bullets, and electric tasers. In some cases, riot squads may also use Long Range Acoustic Devices, water cannons, armoured fighting vehicles, police dogs or mounted police on horses. Officers performing riot control typically wear protective equipment such as riot helmets, face visors, body armor (vests, neck protectors, knee pads, etc.), gas masks and riot shields. However, there are also cases where lethal weapons are used to violently suppress a protest or riot, as in the Boston Massacre, Haymarket Massacre, Hungarian Revolution of 1956, Kent State Massacre and Tienanmen Square Massacre.
http://en.wikipedia.org/wiki/Riot_control