An unknown number of people were reported injured after a crowd of pro-Russian protesters stormed into the regional Prosecutor's Office in Donetsk, east Ukraine.

Clashes ensued after rebels first attempted to break into the building.

They pelted police with Molotov cocktails and stones, while security forces used tear gas and stun grenades, according to Ukraine's 24 Channel.

The activists accused personnel members inside the office of siding with the Kiev government, labeling them "fascists".

Police officers seem to have been helpless in the face of the hundreds-strong multitude which took their weapons and shields while storming the prosecution building.

After the office was taken over, a flag of the Donetsk People's Republic, which rebels proclaimed in April, was hoisted over it.

Shots were heard in the building after it was seized, and reports suggest both police and demonstrators were among the injured.

On Thursday, Ukraine's interim President Oleksanr Turchynov admitted Kiev's army was "helpless" in the east and south of Ukraine, where it started in April a so-called "ant-terrorist" operation against pro-Russian separatists.

Over a few weeks protesters across Ukrainian regions close to the Russian borders have occupied government buildings in a number of towns and cities.

They are adamant that a referendum will be held on whether their territories should join Russia and that the Kiev government must resign.