Western allies fear Russian forces intend to seize control of a large part of Ukraine in a drive to install a pro-Moscow puppet regime in Kyiv.
The attack, described by Boris Johnson as a "vast invasion", began early on Thursday with a series of precision missile strikes and air strikes followed by ground troops crossing the border.
The first targets included Ukrainian air defences, designed to ensure Russia's aerial superiority in the conflict.
Western officials have reported forces moving crossing over from Belarus, where they have been engaged in large scale exercises, and from Russian-occupied Crimea.
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They are also said to have moved into the Donbas region of eastern Ukraine where President Vladimir Putin has recognised to the two breakaway "republics" of Donetsk and Luhansk.
So far, officials say that it is unclear whether he intends to take control of the entire country.
However they believe that his objectives include the capital Kyiv and the port city of Odesa as well as joining up Crimea, which he seized in 2014, with the Donbas.
Ukrainian forces have been putting up resistance, with reports that at least one Russian warplane has been brought down.
However analysts believe that the combination of aerial superiority, precision munitions and artillery firepower gives Moscow's forces a marked military advantage.
The Russians are able to mass their troops in particular locations supported by overwhelming firepower whereas the stretched Ukrainians are trying to defend a large territory.
Ahead of the invasion there were reports of cyber attacks overnight on a number of Ukrainian financial and government institutions, amid widespread suspicions that they were carried out in support of the attack.
Officials believe that Russian special forces are now active inside Ukraine conducting operations to soften resistance in ahead of the main conventional forces advance.Video LoadingVideo UnavailableClick to playTap to playThe video will auto-play soon8CancelPlay now
There are particular concerns now about the prospect of an assault on Kyiv, a city with a population of more than 2.8 million.
In the past, such as in the Chechen capital Grozny, Russian forces having shown they are willing to use overwhelming firepower if they cannot achieve their objectives quickly, potentially resulting in large-scale civilian casualties.