Football's a game of tribes, passions and loyalties that frequently boils over. Here's a collection of some truly remarkable images featuring flashpoints between ultras and police from both outside and inside stadiums around the world.
Newcastle United fan punches a horse
Legia Warsaw Ultras are intense.
Russian and English hooligans clash at Euro 2016
England fans at Euro 2016
The Old Firm Derby at Celtic Park between Rangers and Celtic
Arsenal vs. Millwall in 1982 at Highbury.
The Boca Juniors Barra Brava at La Bombonera in Buenos Aires.
Police use a hose on supporters in Argentina.
A helicopter is used to disperse the crowd at the Africa Cup of Nations.
Legia Warsaw ultra uses pepper spray on a steward.
Russian hooligans clash with police.
The Eternal derby in Belgrade
The Intercontinental Derby in Istanbul
Police clash with hooligans
A Feyenoord Ultra before a match against Rotterdam.
Olympiakos Ultras storm the gate
An Ultra submerged in smoke
Saint-Etienne supporters in Manchester
CSKA Moscow supporters brave the elements.
Palmeiras Ultras clash with police.
Ultras and Police Clash in the Netherlands
Man running from police
Besiktas Ultras Throw Chairs
Hooligans Attack a Police Officer
Police Search a Child
The Tyne-Wear derby in England
Sao Paulo supporters
Saint-Etienne supporters take control