Bulgaria's ruling party insisted on Thursday Prime Minister Boyko Borisov's government would not resign after two months of protests against his perceived tolerance of corruption erupted into the most violent day yet.

Several thousand demonstrators gathered outside parliament on Wednesday as it began its autumn session and the rally took a violent turn.

Over the course of the day there were scores of injuries and arrests in scenes unprecedented in Bulgaria in recent years, with police using stun grenades and tear gas to disperse the crowds while some protesters threw powerful smoke bombs.

Police say 126 people were arrested, among them more than 60 football "ultras" who had previous criminal records.

"We will not resign after these excesses," MP Toma Bikov from Borisov's centre-right GERB Party told parliament on Thursday.

"This would mean that every future government could be brought down by representatives of the criminal contingent," Bikov said.

The wave of protests was sparked in early July by several incidents revealing high-level corruption and perceived government protection of shadowy oligarchs.

The demonstrators want Borisov and Chief Prosecutor Ivan Geshev to resign.

They accuse Geshev of failing to punish real criminals and instead using his powers to attack the government's political opponents.

Almost 200 hurt 

On Wednesday powerful smoke bombs, firecrackers and other devices were thrown at police cordons encircling parliament prompting anti-riot forces to disperse the rally by force.

Health officials said almost 200 people had to receive medical attention over the course of the day, including 120 police officers.

Several journalists were also injured by police, including an AFP photographer.

After dispersing Wednesday night's rally, police moved early Thursday to lift two protest blockades of key downtown crossroads in Sofia, saying that dangerous objects, including a Molotov cocktail, were found hidden in the tent camps.

Some protest organisers have expressed dismay at Wednesday's violence and called for calm.

Borisov, who has been in power almost without interruption for more than a decade, has so far refused to resign before his third term expires in March next year.

In an attempt to appease protesters, the veteran premier has already sacked several key ministers and has put forward a proposal for a new constitution, but protesters have dismissed these moves.

The proposal has little chance of success as Borisov lacks the two-thirds majority in parliament needed to push it through and analysts see it as an attempt to win time and cling to office.

Thirteen years after joining the EU, Bulgaria remains its poorest and most graft-ridden member, according to Transparency International's corruption perceptions index.