Georgia's prisons minister resigned Wednesday over videos showing the alleged beating and rape of convicts that caused outrage ahead of bitterly-contested elections in the ex-Soviet state.

Some of the graphic video footage, aired late Tuesday by opposition television station TV9, showed a weeping half-naked male prisoner at a jail in Tbilisi begging for mercy before apparently being raped with a stick.

"What happened in penitentiary number 8 is horrific and I have submitted my resignation to the prime minister," minister Khatuna Kalmakhelidze said in televised comments.

Another video released by the interior ministry showed prison guards brutally kicking an inmate.

A Georgian government statement said that 15 alleged perpetrators had already been arrested and claimed that the videos were staged by paid provocateurs paid to discredit the authorities ahead of the polls.

"It should be noted that these were premeditated crimes and there is evidence that those who arranged, conducted and recorded these abuses were paid to do so," the government statement said.

The videos have caused a scandal ahead of the October 1 polls which will see the ruling party of President Mikheil Saakashvili facing a major challenge from an opposition bloc led by billionaire tycoon Bidzina Ivanishvili who has vowed to oust the government.

Several hundred people staged an angry protest and blocked one of the capital's main streets, some carrying hand-drawn pictures of prisoners being beaten and others holding photographs of relatives allegedly abused in jails.

"I am protesting against torture in prisons, which are under the strict control of the authorities. The authorities are responsible for what's happening there," one demonstrator, bank worker Mikheil Javakhishvili, told AFP.

"All of Georgia must take to the streets in protest against this horror," said another protester, Sophia Gabichvadze.

Saakashvili said in a statement that Georgia "will not tolerate such behaviour -- in its prisons or anywhere else".

"We have not overcome years of impunity and lawlessness to allow some people, whether they are from the criminal world or, worse, from the penitentiary system itself, to commit such crimes," he said.

The interior ministry said in a statement that the alleged abusers were prison officers who "exercised inhumane treatment against prisoners and made video records according to a previously elaborated plot".

One of the videos purports to show an inmate tied to the bars of a cell with what appears to be a stick inserted in his anus.

A voice repeatedly asks him: "What are you?" The alleged convict replies that he is a crime boss.

Next month Saakashvili's governing party faces its toughest electoral battle since coming to power after the 2003 Rose Revolution, against a revitalised opposition led by tycoon Ivanishvili whose TV station aired some of the prison videos.

Tensions have escalated, raising fears of confrontation in the small Western-backed Caucasus republic with a recent history of political unrest.

The opposition has often accused the government of being responsible for maltreatment of convicts.

But the government statement said that "all necessary action will be taken to ensure that such abuses never again occur in Georgian prisons".