Russia's invasion of Ukraine is approaching the outskirts of Kyiv.
It comes after Moscow unleashed air strikes on cities and military bases, as well as sending in troops and tanks from three sides in an attack that could rewrite the global post-Cold War security order.
On Friday, explosions sounded before dawn in Kyiv with gunfire later heard near the government quarter as Western leaders scheduled an emergency meeting.
At the same time, Ukraine's president Volodymyr Zelensky pleaded for international help to fend off an attack that could topple his democratically elected government and cause massive casualties.
"If you don't help us now, if you fail to offer a powerful assistance to Ukraine, tomorrow the war will knock on your door," Mr Zelensky said.
Mr Zelensky said in a video address that 137 "heroes", including 10 military officers, had been killed and 316 people wounded.
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He concluded an emotional speech by saying that "the fate of the country depends fully on our army, security forces, all of our defenders".
His comments came after he cut diplomatic ties with Moscow, declared martial law and ordered a full military mobilisation that would last 90 days.
Also on Friday, the Ukrainian military said a group of Russian spies and saboteurs had been seen in a district of Kyiv about three miles north of the centre.
Police also told people not to exit a subway station in the city centre because there was gunfire in the area.
Elsewhere in Kyiv, soldiers established defensive positions at bridges, and armoured vehicles rolled down the streets, while many residents stood uneasily in doorways of their apartment buildings.
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Earlier, the military said that Russian forces had seized two Ukrainian military vehicles and some were heading towards the city to try to infiltrate under the guise of being locals.
US secretary of state Antony Blinken said Kyiv "could well be under siege" in what US officials believe is a brazen attempt by Russian President Vladimir Putin to install his own regime.
The assault, anticipated for weeks by the US and Western allies, amounts to the largest ground war in Europe since the Second World War.
The invasion began on Thursday morning with a series of missile strikes on cities and military bases.
This was quickly followed with a multi-pronged ground assault that rolled troops in from several areas in the east as well as from the southern region of Crimea, which Russia annexed in 2014, and from Belarus to the north.
While some nervous Europeans speculated about a possible new world war, the US and its NATO partners have shown no indication they would send troops into Ukraine, fearing a larger conflict.