Yes it is possible for PC just enter a police or army car and press caps lock key the vigilante mission will start and you will be prompted to chase a criminal on map.
Grand theft auto 4 cheat for police bike
Start Killing in Shoreside Vale
yes they will chase you
You could use the raise wanted level cheat or get into a police chase and kill a whole bunch of people and cars.
No you cannot be a police officer in Grand Theft Auto, but you can in Saints Row: The Third
it is ifirstdoz
On Grand Theft Auto IV no you cannot but you can do vigilante missions if you manage to obtain a police car