There is street fighting in Kharkiv after Russian troops entered the city in the earlier hours of today.
It comes after a huge explosion at a gas pipeline on the edge of the city.
Russian troops approached the city, about 12 miles south of the border from Russia, shortly after the invasion was ordered by Vladimir Putin on Thursday morning.
Until today the heavily armed soldiers had remained on the outskirts of Kharkiv without trying to enter it.
Now Oleh Sinehubov, head of the city's regional administration, has warned civilians to stay inside and not to leave their homes as their troops are fighting Russians on the streets.
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Videos posted on social media showed Russian vehicles, including armoured tanks, moving against the city, the second largest in the under siege nation with around 1.5million inhabitants, as well as a light vehicle burning in the streets.
Pictures on social media show the extent of the street battle as a large explosion was captured on camera by terrified residents.
President Volodymyr Zelensky's office said enemy forces had blown up a gas pipeline near Kharkiv, prompting advice for people to protect themselves from smoke by covering windows with a damp cloth or gauze.
An apartment building is also thought to have been damaged by a missile strike.
Earlier unsubstantiated claims by the Russian Defence Ministry claimed the 302nd anti-aircraft missile regiment of the Ukrainian Armed Forces in Kharkiv region had laid down its arms.
According to military sources, 471 people had been detained, although it was not confirmed.
It comes after Russia unleashed Putin's fury on Ukraine, targeting its airfield and fuel facilities
Huge explosions lit up the sky early on Sunday south of capital Kyiv, where people hunkered down in homes, underground garages and subway stations in anticipation of a full-scale assault by Russian forces.
Flames billowed into the sky before dawn from an oil depot near an air base in Vasylkiv, where there has been intense fighting, according to the town's mayor.
More than 150,000 Ukrainians have fled for Poland, Moldova and other neighbouring countries, and the United Nations warned the number could grow to four million if fighting escalates.
The country's men have been banned from leaving as Ukraine's leaders need boots on the ground to repel the Russian threat.
Earlier, Boris Johnson praised Ukrainians for "fighting heroically" as allies unveiled new measures to hit Russia's financial system.
