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.

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 Caracas (Carlos 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 Kabul (Omar 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.

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".

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.

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 formed (Olivia Harris/Reuters)