TBILISI: Hundreds of people rallied in Georgia on Friday in a fourth day of protests over brutality in state prisons, a bout of unrest that could damage prospects for President Mikheil Saakashvili's party to come out on top in elections on Oct. 1.

The protests, which broke out after two television channels that back Saakashvili's opponents showed footage of prison abuses, have saddled him with an unexpectedly tough election battle against a coalition led by a billionaire businessman.

About 500 people gathered in the capital Tbilisi on Friday, markedly fewer than before the resignation of the prisons and interior ministers in mid-week. But analysts said public anger over violence in jails threatens the ruling National Movement party's chances of winning the coming parliamentary vote.

The opposition Georgian Dream coalition led by billionaire Bidzina Ivanishvili was 20 percentage points behind Saakashvili's party before the protests.

"The ruling party will (now) get fewer votes than had been expected, of course, but it's difficult to predict how big the decline will be," said Gia Nodia, an independent analyst.

"It's not clear whether Georgian Dream will get these votes or another opposition party."

He said the exit of the ministers and Saakashvili's moves - he quickly denounced the abuses - had largely been effective.

But Shorena Shaverdashvili, editor-in-chief of the independent Liberal magazine, said Saakashvili's party could now lose an election it had seemed certain to win in the former Soviet republic.

"If elections are free and fair, the ruling party has no chance of winning, but a lot will depend on the voter turnout," Shaverdashvili said. "The system has been shaken. Even Saakashvili himself admitted that it was a systemic failure."

Hours after the release of the prison video, Saakashvili promised to punish those responsible and seek radical reforms of the jail system, asking policemen to take over prison guard duties while reforms were being worked out.

Saakashvili's government says the video, which shows guards beating, punching and humiliating prisoners, as well as inmates being raped with objects, was staged and recorded by guards who were bribed by "politically motivated persons".

The head of the Tbilisi prison, his two deputies and several prison guards were arrested, while international organisations and human rights groups called for a prompt investigation.

Ivanishvili, his fortune estimated by Forbes magazine at $6.4 billion, owns one of the broadcasters that showed the film.

A once-reclusive tycoon whose wealth equals nearly half Georgia's economic output, Ivanishvili launched his political movement last year and has campaigned on calls for Saakashvili to resign.

Saakashvili became the West's political darling when he rose to power after the bloodless "Rose Revolution" that toppled Eduard Shevardnadze, a former Soviet foreign minister, in 2003.

But opponents have accused Saakashvili of curbing political freedoms and criticised him for leading Georgia - a country of 4.7 million people on a transit route for oil and gas supplies across the volatile Caucasus region - into a brief but disastrous war with Russia in August 2008.