US police are under pressure not only for the killing of an unarmed teenager in Ferguson, Missouri, but also for the military-style response to the sometimes violent protests that followed.

The sight of police in camouflage gear brandishing assault rifles, backed up by armoured vehicles was a reminder that some US police departments have acquired military-surplus hardware from wars abroad.

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Police armed with a range of weaponry attempt to control demonstrators protesting the killing of teenager Michael BrownScott Olson / Getty Images
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A police officer raises his weapon at a car speeding in his general direction as a group of demonstrators stands on the pavementLucas Jackson / Reuters
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Armed police are silhouetted against clouds of tear gas as they advance towards protesters in Ferguson, MissouriScott Olson / Getty Images
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Police officers react to the movements of a group of demonstratorsLucas Jackson / Reuters

Many other law enforcement agencies around the world have rules of engagement that allow lethal force to be used relatively freely. But for every regulation that gives police wide scope to use firearms, there is another code that sharply limits their use.

Reuters photographers around the world took portraits of police officers, and asked them at what point are they legally permitted to use force to control crowds.

Venezuela's Interior Ministry decrees that, when peaceful methods of resolution have failed, police must warn violent demonstrators that there will be a "progressive, differentiated use of force". While no firearms must be carried for peaceful demonstrations, when things turn violent, the emphasis is on avoiding harm to children, pregnant women and the elderly, and no force is to be used on those who avoid violence or are withdrawing from the scene.

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Venezuelan national police officers Bogado and Bello pose with their riot equipment, next to a mannequin in uniform during a government Christmas fair in CaracasCarlos Garcia Rawlins/Reuters

Afghanistan's police, often themselves the target of armed attacks, are officially authorised to respond with weapons "and explosives" against a group of people only if it has ... disturbed security by means of arms, and if the use of other means of force ... has proved ineffective". Afghan police are required to give no fewer than six warnings – three verbal and three warning shots – before using force in this situation.

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Afghan policemen Shir Agha, 24, Shkib, 24, Qayam, 22, Farid Ahmad, 26, and Sobhan Ullah, 22, pose for a photo in KabulOmar Sobhani/Reuters

Mexican and Indian riot police follow defined escalation protocols that go from verbal warnings to physical constraint, tear gas, water cannon or pepper spray, rubber bullets or baton rounds, and then use of firearms.

Yet while Mexican police commanders can decide when to escalate, India's Rapid Action Force requires approval from an on-the-spot magistrate for each new step.

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Members of the Task Force for Mexico City pose for a photograph at their base. In Mexico, "when violent action by a crowd cannot be deterred, a scale of force will be applied progressively consisting of 1. verbal persuasion or deterrence 2. reduced physical movements 3. use of non-lethal incapacitating weapons, and 4. use of firearms or lethal force"Claudia Daut/Reuters
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India's Rapid Action Force (RAF) personnel pose for pictures inside their base camp in New Delhi. In India, the RAF are called on for violent disorder that the police are unable to contain. They require an on-the-spot magistrate's consent and must issue a warning before each escalation of the use of force, from verbal warning to water cannon and tear gas, then to rubber bullets or baton rounds, and then to firearmsAdnan Abidi/Reuters

Many countries spell out that any use of firearms is a last resort, though this can be defined many ways.

Britain, Serbia, Bosnia and the Philippines allow guns to be fired only if a life is at risk.

Britain stands out for its insistence that "individual officers are accountable and responsible for any use of force and must be able to justify their actions in law".

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Metropolitan Police constables Ben Sinclair and Karen Spencer pose wearing their beat uniforms, in London. In Britain, "lethal or potentially lethal force should only be used when absolutely necessary in self-defence, or in the defence of others against the threat of death or serious injury"Paul Hackett/Reuters
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Serbian police officers of the Special Anti-Terrorist Unit pose at their base outside Belgrade. In Serbia, police may use measures ranging from batons to special vehicles, water cannon and tear gas on groups of people who have gathered illegally and are behaving in a way that is violent or could cause violence, but they may use firearms only when life is endangeredMarko Djurica/Reuters
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Members of a Bosnian Special Police Support Unit pose for photo in front of their base in Zenica. In Bosnia, police are permitted to use force ranging from batons to chemical irritants, water cannon, "binding agents, special firearms and explosive devices", following a warning, but only when other methods of control have proved ineffective, and not against the young, old or disabled unless these use firearms. The method must be "proportional to the resistance or violence coming from the person on whom the force is used"Dado Ruvic/Reuters
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Members of the Philippine National Police Special Weapons and Tactics (SWAT) team pose outside a police station in Manila. In the Philippines, the use of extreme force against a suspect is allowed only if the police officer's life or that of the victim (of the suspect) is in imminent dangerRomeo Ranoco/Reuters

Many West European countries allow firearms to be used "where necessary" to detain suspects or to prevent a serious crime.

Police at the extraterritorial United Nations buildings in Geneva are not subject to Swiss law but still conform to local police rules. These rules, like those governing police in Italy, Austria and Belgium, specify that the use of force must be "proportionate".

In Belgium, human rights monitors say, this means firearms can never be used for crowd control.

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Members of the United Nations security forces pose in front of the UN European headquarters in Geneva. They follow the local cantonal police rules, which say that "the use of weapons, proportionate to the circumstances, is authorised as a last resort" but should "avoid serious injury whenever possible", and that "the use of a firearm is preceded by a warning – if circumstances permit". The United Nations Office in Geneva is considered ex-territorial and is not under the jurisdiction of the host country SwitzerlandDenis Balibouse/Reuters
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Italian Carabinieri pose in front of St Peter's Basilica as a helicopter flies overhead. In Italy, police and the paramilitary Carabinieri follow guidelines that say the use of weapons is allowed only in the line of duty, when it is an "unavoidable necessity to overcome resistance, stop violence, or prevent a [serious] crime", and that the response must be proportionate to the situationAlessandro Bianchi/Reuters
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Austrian police officers pose in various uniforms in front of a water cannon at their headquarters in Vienna. The uniforms include (L to R) light demonstration uniform, full combat adjustment for life threatening mission including gun (not used for riots in Austria), normal daily life uniform of commander, uniform of riot police officer, and normal uniform of police officer. In Austria, the use of lethal force is permitted to tackle rioting or to detain a dangerous suspect, but only when less dangerous methods "appear inappropriate or have proved to be ineffective", and with the aim of avoiding serious injury where possible. The use must be proportionate, and be preceded by a warningLeonhard Foeger/Reuters
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Members of Belgium's special forces pose for pictures at their headquarters in Brussels. Human rights monitors say police in Belgium are legally entitled to use proportionate force, after a warning, where there is no other means to achieve a legitimate objective. They say police may use firearms in self-defence, to confront armed perpetrators, or in defence of persons or key facilities, but never for crowd controlYves Herman/Reuters

Malaysia's Federal Reserve Unit, the main riot force, is permitted to use firearms only when protesters are using them, but it is in a fortunate position. Its deputy superintendent, Kulwant Singh, says that "firearms have not been used in the 59 years since the FRU was formed".

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Malaysia's public order police, the Federal Reserve Unit (FRU), pose for photographs wearing riot control equipment at their headquarters in Kuala Lumpur. The FRU are only permitted to use firearms in cases where the protesters are using firearms. Firearms have not been used in the 59 years since the FRU was formedOlivia Harris/Reuters