The 30-day measure allows authorities to impose restrictions on movement, block rallies and ban political parties and organisations “in the interests of national security and public order”.
Ukraine's parliament has approved a nationwide state of emergency to come into force at midnight on Wednesday into Thursday. The move was proposed earlier by President Zelenskyy and backed by the National Security Council.
The Kremlin said on Wednesday night that rebel leaders in eastern Ukraine have asked Russia for military help to fend off Ukrainian "aggression". Washington called it a classic "false flag" operation designed to provoke a pretext for Moscow to invade.
It comes as the region braced for further confrontation after Vladimir Putin received authorisation to use military force in eastern Ukraine and the West responded with sanctions. An emergency EU summit of the bloc's 27 national leaders is to convene on Thursday night in Brussels.
The United Nations Secretary General Antonio Guterres told the UN General Assembly on Wednesday that the "world is facing a moment of peril", as he again accused Russia of violating the UN charter.
Ukraine's Foreign Minister Dmytro Kuleba called for an immediate "severe, swift and decisive response" against Moscow, warning that failure would "mean a total bankruptcy of the international security system and international institutions".
Kyiv has announced a call-up of reservists and warned its nationals in Russia to leave the country.
The Russian ambassador to the UN hit back, Vasily Nebenzya telling the assembly that Ukraine was to blame for waging war on its own citizens. Earlier, a close ally of President Putin in the Russian parliament, the Duma, described new EU sanctions as "worthless".
Follow all of Wednesday's key developments here
US Secretary of State Antony Blinken announced that a meeting with his Russian counterpart, Sergei Lavrov, scheduled on 24 February has been cancelled, arguing Moscow is not serious about diplomacy.
It comes after the US, UK, and EU unveiled sanctions against Russian individuals and entities in response to Putin's recognition of the so-called Donetsk People's Republic and Luhansk People's Republic — two separatist-held areas in eastern Ukraine — and his announcement that troops would be sent in on "peacekeeping duties."