President Petro Poroshenko said that he is introducing martial law across Ukraine for 30 days, starting on Thursday.
The move comes one day after Russian ships fired at and seized Ukrainian vessels in the Black Sea, triggering a sharp escalation in tensions between the neighbouring countries. Several Ukrainian sailors were injured, and 24 were detained.
The international community has condemned Russia's moves, with the European Union and Nato calling for restraint.
Russia warned Ukraine of "serious consequences" if it continues what Moscow describes as stoking conflict.
An emergency session of the UN Security Council will be held today to discuss the crisis.