The Ukrainian government has brought back military conscription under a presidential decree as the crisis in the east of the country intensifies.
Clashes have broken out between pro-Russian protesters and police as May Day demonstrators tried to storm official buildings.
Sky's Chief Correspondent Stuart Ramsay, is in central Donetsk, in eastern Ukraine, where he said protests had turned "very, very violent".
"Protesters have been to a number of buildings but they ended up trying to take the office of the prosecutor, who is the man attempting to issue arrest warrants for some of the pro-Russian militia and their supporters," he said.
"They tried to storm the building, which is being defended by riot police, and for the first time I've seen, the officers tried to fight back
"A lot of tear gas has been fired and the building is now being pelted with rocks.
"I saw a number of police officers being dragged away, many of them with quite serious injuries and injured people on both sides have been brought down the steps of the building.
"I also saw a number of officers, a group of perhaps 30 or so, running from the scene."
It comes after Russia's military attache to the Ukrainian capital Kiev was reportedly detained on suspicion of spying.
The diplomat was held after allegedly undertaking "intelligence activities", according to the Interfax-Ukraine news agency.
Citing the Ukrainian foreign ministry, it said the official had been ordered to leave the country.
he move comes after Ukraine's armed forces held military exercises in central Kiev overnight.
Interim president Oleksandr Turchynov said soldiers were on "full combat alert" over a possible invasion by Russian troops massed on the border.
Russia responded by demanding Ukraine stop issuing "militaristic statements".
The country's foreign ministry said: "We insist that Kiev immediately ceases the bellicose rhetoric, which is aimed at intimidating its own population."
Ukraine has accused Russia of orchestrating the fall of eastern towns and cities to pro-Russian separatists over the past month.
Gunmen wearing masks and military fatigues have taken control of key buildings in places including Donetsk, where a 'People's Republic' has been declared.
They also seized important sites in Luhansk, Horlivka and Alchevsk.
However, Russia denies it has any plans to invade Ukraine after its annexation of the Crimean peninsula in March.
The crisis was triggered when ousted Ukrainian President Viktor Yanukovych abandoned an agreement on closer trade ties with the EU in November.
Violent clashes erupted across the country when he signed a deal with Russian President Vladimir Putin the following month.
More than 100,00 people have turned out for a May Day march through Moscow's Red Square as the annexation of Crimea has triggered a surge in patriotism and support for Mr Putin.
"Putin is right", "Proud of the country" and "Let's support decisions of our president" read banners carried by the demonstrators.
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