Putin announced the action during a televised address early on Thursday (24 February) morning, saying the move was a response to threats from Ukraine.
He said Russia does not have a goal to occupy Ukraine, but the responsibility for bloodshed lies with the Ukrainian “regime”.
But what is martial law? And what exactly does it mean for people living in Ukraine?
Here is everything you need to know about it.
What is martial law?
Declaring a state of martial law means granting military bodies and armed forces authority over an area.
It is usually implemented when civil authorities - like police forces and other emergency services - become overwhelmed during emergency situations.
Designed to be imposed only on a temporary basis, martial law has been brought in around the world in the past, in response to foreign invasions, riots and natural disasters.
Military forces may be deployed for various purposes such as to stop looting, maintain order, impose a curfew, and secure buildings of the government.
Typically, martial law is accompanied by the introduction of curfews, and the suspension of civil laws in favour of military law and the imposition of military justice on civilians.
This means that civilians found defying martial law may be subjected to court-martial.
While in the current case of Ukraine it has been declared to help the military defend territories now being encroached upon by Russian forces, history has shown that martial law declarations can also be used for nefarious purposes.
Martial law can be used by governments to enforce their rule over the public; martial law was declared in response to the Tiananmen Square protests of 1989 in China, for instance, allowing the military to clear out demonstrators.
What does it mean for Ukraine?
The exact specifics of martial law differ from country to country, and the restrictions in place under such declarations can be tweaked and set by leaders at any given time.
For example, the last time martial law was declared in Ukraine was in 2018, in 10 regions mostly bordering Russia in an attempt to strengthen the country’s defences against flaring tensions.
For 30 days, all Russian men between 16 and 60 were barred from entering Ukraine, for fear that “private” armies could form within its borders.
President Zelensky is yet to define exactly what restrictive measures Ukraine will be under during the current period of martial law, but they will likely be set out in due course.
Zelensky has called for people with military experience to “join the defence of Ukraine” and asked for blood donations to help wounded soldiers.
He has also said: “We will give weapons to anyone who wants to defend the country. Be ready to support Ukraine in the squares of our cities.
“We will lift sanctions on all citizens of Ukraine who are ready to defend our country as part of territorial defence with weapons in hands.”
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