The move, decreed by interim president Oleksandr Turchynov, came after insurgents tightened their grip over more than a dozen eastern cities and towns. Around 300 militants hurling petrol bombs and bricks stormed the six-storey prosecutor's building in Donetsk, beating up outnumbered riot police and stripping them of their shields and batons.
Ukrainian media reported that a prosecutor's office in the town of Horlivka and a police station in Krasnoarmiysk were also overrun. The violence took place as mass pro-Russia rallies were held in Donetsk and in annexed Crimea.
Kiev's Western-backed government has already admitted its security forces are helpless to halt the expanding rebellion it accuses Moscow of masterminding. Turchynov yesterday accused law enforcement units in the east of "inaction" or even working with the rebels in an act of "treachery".
He also put Ukraine's current army of 130,000 on "full combat alert" because of fears an estimated 40,000 Russian troops massed on the border for the past two months could invade.
In his conscription order today for Ukrainian male reservists aged 18-25, Turchynov said his government was trying to counter "the deteriorating situation in the east and the south".
The mounting insurgency and building seizures "threaten territorial integrity," a statement from his office said. Russia's foreign ministry said any effort by Kiev to intensify its military operation "against its own people" in the east could have "catastrophic consequences".
In another dramatic development, Kiev overnight ordered out a Russian diplomat arrested for espionage, risking a tit-for-tat response from Russia.
Amid the spiralling crisis, Germany stepped up its appeal to Russian President Vladimir Putin to help free seven OSCE inspectors held in the town of Slavyansk by the rebels -- four Germans, a Pole, a Dane and a Czech. German Chancellor Angela Merkel "reminded President Putin of Russia's responsibilities as an OSCE member and called on him to use his influence," Merkel's spokesman said.
The Kremlin said both leaders emphasised the "mediating potential of the OSCE" in calming the crisis in Ukraine. Putin reiterated his call for Kiev to end its military operation trying to counter the pro-Russian rebellion.
That drew an incredulous reaction from the White House. A spokesman said: "That was a rather remarkable statement... (that) called on Ukraine to remove its forces from its country, which is preposterous, if you think about it."