Ukrainian airspace was closed to civilian use on Thursday after Russia launched a long-expected military intervention.
The measure was taken due to the "high risk" to civil aviation, said the Ukrainian State Air Traffic Service.
Separately, Russia announced the closure to civilian flights of its airspace on the western border with both Ukraine and Belarus.
A real-time flight tracking app "Flightradar24" screen shows empty space as all planes diverting around Ukraine on Thursday.
Early Thursday Russia announced a military intervention in Ukraine, drawing international condemnation and vows of tougher sanctions on Moscow.
Explosions were reported in several Ukrainian provinces, including the capital Kyiv, and tanks reportedly crossed the border from Belarus into Ukraine.
In recent months Russia had amassed over 100,000 Russian troops around Ukraine, but repeatedly denied any intention of invading.