The move, announced in a decree, came as pro-Russia militants seized the regional prosecutor's office in the eastern city of Donetsk.
Ukraine blames Russia for organising the seizures of a number of offices in the east, a claim Moscow denies.
Some 40,000 Russian troops are stationed near the Ukrainian border.
Mr Turchynov admitted on Wednesday that his forces were "helpless" to quell the unrest in some parts of the east, saying the goal was now to prevent it from spreading.
He also said Ukraine was on "full combat alert", amid fears that Russian troops could invade.
On Thursday, his office said in a statement that conscription was being introduced "given the deteriorating situation in the east and the south... the rising force of armed pro-Russian units and the taking of public administration buildings... which threaten territorial integrity". » | Thursday, May 01, 2014
