Russian troops have launched their attack on eastern Ukraine just hours after President Vladimir Putin announced a "military operation".

Large explosions were heard before dawn in Kyiv, Kharkiv and Odesa as world leaders decried the start of an Russian invasion that could cause massive casualties and topple Ukraine's democratically elected government.

The Russian military said it has knocked out Ukraine's air defence assets and airbases, AP reported.

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Kyiv Mayor Vitaly Klitschko has advised residents to stay home unless they are involved in critical work and urged them to prepare go-bags with necessities and documents if they need to evacuate.

Meanwhile, the mayor of Kharkiv also warned citizens not to leave their homes.

"Please do not leave your homes today. Due to the complicated situation, schools, kindergartens and other institutions do not work today — until the situation becomes clearer," Mayor Igor Terekhov said in a Facebook post.

Australia's Department of Foreign Affairs and Trade has urged Australians in Ukraine to "shelter in place until it's safe to depart".

China and US have also advised their citizens to shelter in place.

However, pictures emerging from Ukraine have shown residents fleeing the capital in droves.

The traffic appears to be significantly banked up.

Residents can also be seen sheltering in underground train stations.

Air raid sirens have also sounded in Ukraine's capital Kyiv.

9News reporter Carrie Greenbank was live on air when the air raid sirens began while CNN reporters have also confirmed hearing air raid sirens sounding for several minutes.

"We heard pretty consistent ones [explosions] maybe an hour ago but they have certainly died off," Greenbank said.

"We've got a siren that just started.

"We've also just heard the bells tolling at St Michael's Golden Dome Orthdox Church and that was going to be done when Russia invaded Ukraine."

https://twitter.com/CGreenbank9/status/1496702952165490691

Greenbank said they hadn't been told about sirens and what they mean.

"There hasn't been a lot of talk about what to do if you hear a siren like this," she said.

CNN reporters said the mood is "remarkably calm" despite the explosions and air raid sirens.

After the initial explosions in Kyiv, people could be heard shouting in the streets, AP reported.

Then a sense of normality returned, with cars circulating and people walking in the streets as a pre-dawn commute appeared to start in relative calm.

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Martial law declared in Ukraine

Ukrainian President Volodymyr Zelenskyy has introduced martial law across the country.

In a video address posted on his Facebook, Mr Zelenskyy urged people to stay calm.

"Russia conducted strikes on our military infrastructure and our border guards. There were blasts heard in many cities of Ukraine," Mr Zelenskyy said.

"We're introducing martial law on the whole territory of our country.

"Stay at home if you can. We are working. The army is working.

"The whole sector of defense and security is working.

"No panic. We are strong. We are ready for everything.

"We will win over everybody because we are Ukraine."

Mr Zelenskyy also added the US had already begun coalescing international support.

'Consequences they have never seen'

In a televised address today, Mr Putin said the invasion comes in response to threats coming from Ukraine.

He warned other countries that any attempt to interfere with the Russian action would lead to "consequences they have never seen".

Mr Putin said Russia doesn't have a goal to occupy Ukraine, and that the responsibility for bloodshed lies with the Ukrainian "regime".

He accused the US and its allies of ignoring Russia's demand to prevent Ukraine from joining NATO and offer Moscow security guarantees.

He said the Russian military operation aims to ensure a "demilitarisation" of Ukraine.

Mr Putin said that all Ukrainian servicemen who lay down arms will be able to safely leave the zone of combat.

Just hours ago, Mr Putin issued a ban on civilian air traffic in airspace over eastern Ukraine.

The UN Security Council has held a second emergency meeting on the unfolding situation in Ukraine.

US President Joe Biden condemned an "unprovoked and unjustified attack by Russian military forces".

Ukrainian president vows to 'fight back'

Mr Zelenskyy vowed earlier today his country would "fight back" against the Russian invasion.

In an emotional address to the nation, Mr Zelenskyy said he had pleaded for peace adding that he tried to contact Mr Putin but the Russian leader would would not accept the call.

Mr Zelenskyy rejected Moscow's claims that his country poses a threat to Russia and lamented that a Russian invasion would cost tens of thousands of lives.

Mr Zelenskyy said "nearly 200,000 soldiers" were stationed on the Ukrainian border, along with "thousands of combat vehicles".

"The people of Ukraine and the government of Ukraine want peace," he said in Russian, hours after declaring a nationwide state of emergency.

"Today I initiated a telephone conversation with the President of the Russian Federation. Silence. Although there should be silence in the Donbas," Mr Zelenskyy said.

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Meanwhile, the heads of the self-proclaimed Donetsk People's Republic (DPR) and Luhansk People's Republic (LPR), two separatist regions in Ukraine, recently recognised by Russia as independent, formally asked Mr Putin for help repelling the aggression of Ukrainian Armed Forces, Kremlin spokesperson Dmitry Peskov told Russian state news agency RIA-Novosti.

Following this formal appeal to Moscow for military support, he Ukrainian parliament approved Mr Zelenskyy's decree that imposes a nationwide state of emergency.

The decree allows Ukrainian authorities to impose restrictions on movement, block rallies and ban political parties and organisations "in the interests of national security and public order".

He acknowledged that his country faces huge odds without membership in a powerful security alliance like NATO.

"We are self-defending ourselves with the support of our partners. But it's Ukrainians who are dying," he said.

Kyiv reported shelling on the frontline in eastern Ukraine, where Russia-backed rebels have held territory since 2014.

The Ukrainian military said one of its soldiers was killed and six more were injured.

Separatist officials reported several explosions on their territory overnight and three civilian deaths.

Ukrainian Foreign Minister Dmytro Kuleba said on Twitter that the West should target Russian President Vladimir Putin where it hurts without delay.

"Hit his economy and cronies. Hit more. Hit hard. Hit now," Mr Kuleba wrote.

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European Union condemns Russian attack

The European Union (EU) has condemned the Russian attack and has called upon the country to "immediately cease the hostilities".

"We condemn in the strongest possible terms Russia's unprecedented military aggression against Ukraine," President Charles Michel of the European Council and President Ursula von der Leyen of the European Commission said in a joint statement.

"By its unprovoked and unjustified military actions, Russia is grossly violating international law and undermining European and global security and stability.

"We deplore the loss of life and the human suffering."

EU leaders are expected to meet later tonight to discuss "further restrictive measures".

'Russia alone responsible for the death and destruction'

US President Joe Biden condemned an "unprovoked and unjustified attack by Russian military forces" in a statement Wednesday evening following explosions in Ukraine.

"President Putin has chosen a premeditated war that will bring a catastrophic loss of life and human suffering," Mr Biden said.

"Russia alone is responsible for the death and destruction this attack will bring, and the United States and its Allies and partners will respond in a united and decisive way."

"The world will hold Russia accountable."

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Mr Biden said that he planned to speak to the American people on Thursday and would announce "further consequences" that the US and its allies planned to impose on Russia over "this needless act of aggression against Ukraine and global peace and security."

"We will also coordinate with our NATO Allies to ensure a strong, united response that deters any aggression against the Alliance," Mr Biden said.

With Associated Press and CNN