Vladimir Putin made the announcement in a television address last night
Ukrainian authorities say the country's capital Kyiv is now under an airborne Russian assault.
Reports say an attack is underway on an airport approximately 40 kilometres outside Kyiv.
There are reports of explosions and shelling in several cities in Ukraine, with regional authorities saying a number of people have died.
Ukraine says more than 40 of its soldiers have been killed, and claims 50 Russian troops have died, although there's no confirmation of that from the Kremlin.
There are reports of civilian casualties.
There are traffic jams as people try to get out of the capital city - while there are reports of queues at shops and petrol stations.
Images show Ukrainians queuing up to join the military or give blood.
This follows Russia's president ordering a "military operation" into the eastern part of Ukraine.
Vladimir Putin claims his orders will "protect people" in breakaway regions - who he says have been subject to bullying and genocide.
Putin has warned other nations not to intervene.
Vladimir Putin made the announcement in a TV address:
NATO's secretary general Jens Stoltenberg says the military alliance is prepared:
Ukraine's President says Russian forces are trying to take Chernobyl, the site of the 1986 nuclear disaster.
Volodimir Zelenskyy says his troops will defend the site so a tragedy can't be repeated.
The Ukrainian President declared martial law earlier, and had this message for citizens:
Ukrainian President Volodymyr Zelenskiy said Russia had carried out missile strikes on Ukrainian infrastructure and border guards, adding martial law had been declared and he had appealed to world leaders to impose all possible sanctions on Russia https://t.co/wmOmsoRl1o pic.twitter.com/eoRg8rGABb
— Reuters (@Reuters) February 24, 2022
Mr. Putin's announcement was made at the same time as a UN Security Council meeting was taking place - urging peace.
Speaking afterwards, UN Secretary General Antonio Guterres appealed directly to the Russian leader:
These people spoke as they fled across the border into Poland:
Natalia Gumenyek is a Ukranian journalist based in Kyiv and says a number of towns are being targetted:
Irish businessman Brendan Murphy is also in Kyiv, and is desperately trying to arrange visas for his Ukranian wife and her family to leave the country, and return to Ireland.
Mr. Murphy says the mood among the people of Ukraine is sombre;
Sky's Deborah Haynes reports from Kyiv:
Huge traffic jams can be seen in the Ukrainian capital:
Traffic jams for miles as people head West out of #Kyiv. Many have their family, pets and belongings with them and emotional conversations on all uncertainty for what will the future hold. #Ukraine pic.twitter.com/avV40NeYLj
— Michael Downey (@mgdowney) February 24, 2022
Professor of Peace Studies at Bradford University, Paul Rogers, expects the West will be hesitant to respond to the invasion with a military retaliation for one very strong reason:
Worldwide political reaction
The US President released a statement in response the latest developments.
Joe Biden says the prayers of the entire world are with the people of Ukraine as they suffer an unprovoked and unjustified attack by Russian military forces.
He added that President Putin has chosen a "premeditated war that will bring a catastrophic loss of life and human suffering" and says the west will respond in a united and decisive way to "hold Russia accountable".
The prayers of the world are with the people of Ukraine tonight as they suffer an unprovoked and unjustified attack by Russian military forces. President Putin has chosen a premeditated war that will bring a catastrophic loss of life and human suffering. https://t.co/Q7eUJ0CG3k
— President Biden (@POTUS) February 24, 2022
EU leaders will meet in Brussels this evening for an emergency summit.
Last night the Taoiseach told his parliamentary party what is being witnessed now is like nothing seen, since World War II.
This afternoon, the Taoiseach said the invasion of Ukraine is an outrageous moral breach of international law.
Micheál Martin said the meeting of EU leaders later - to adopt the most severe package of sanctions against Russia it has ever considered - he hopes would aim to hit Vladimir Putin and his henchmen where it hurts most, by targeting their assets and banks.
The Taoiseach said the EU could not stand by on a conflict that could spread beyond the borders of Ukraine:
Foreign Affairs Minister Simon Coveney says the invasion appears to have been well planned and EU sanctions against Russia will be "increased significantly":
European Affairs Minister, Thomas Byrne, says today's summit of EU leaders is hugely important:
European Commission President Ursula von der Leyen is among the international leaders to condemn Russia's "military operation" in Ukraine:
Journalist with Politico, Suzanne Lynch, says there are very dark days ahead:
Department of Foreign Affairs advice
The Department of Foreign Affairs has issued new advice to Irish citizens in light of the invasion of Ukraine.
The DFA us urging all Irish citizens currently in Ukraine to seek shelter in a secure place, and not move around the country in the coming hours.
The Department also advises against all travel to Ukraine.
Regular updates will be issued on the Department of Foreign Affairs Twitter account.
⚠️ Urgent consular advice regarding Ukraine.
— Irish Foreign Ministry (@dfatirl) February 24, 2022
All Irish citizens in Ukraine should shelter in a secure place. Do not move around the country in the coming hours. Please follow the advice of local authorities.
We will post regular updates on @dfatirl as the situation develops.
Ryanair has suspended all flights to and from Ukraine for at least the next 14 days.
In a statement it said all affected passengers will receive email notices this morning.
All flights to and from the country have been removed from sale for at least the next four weeks until further information becomes available from EU safety agencies.
The airline says it remains committed to its services to and from Ukraine and it looks forward to restoring flight services as soon as its safe to do so.
Our statement on flights to / from Ukraine pic.twitter.com/Dn8KSkhnn9
— Ryanair Press Team (@RyanairPress) February 24, 2022
Stock market
Stock markets around the world have tumbled as Russia announced military action.
The FTSE 100 plunged more than 2.5% on opening.
Moscow's main indices are down as much as 45% - while the rouble's fallen to an all-time low against the dollar and the euro.
Price of oil has surged - with Brent crude costing over $100 dollars per barrel, for the first time since 2014.
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