Thousands of protesters rallied in front of the parliament building in the capital Sofia on Wednesday as debate began on the government's plan for a new constitution. It was one of the biggest protests so far, since demonstrations calling for the resignation of Prime Minister Boyko Borissov and the country's chief prosecutor, Ivan Geshev, started two months ago.

Scuffles broke out between protesters and police, who fired pepper spray and made several arrests. Protesters threw eggs, apples and garbage at the heavily guarded parliament, as the rallies heated up. 

Twenty officers were said to have been injured in the clashes, while 16 people, including protesters, were admitted to hospital, according to emergency services. 

Bulgarians, who have been rallying in Sofia in their thousands since early July, called out what they said was endemic corruption in the country.

They accuse three-times premier Borissov of only serving the interests of private businesses and powerful tycoons in the European Union's poorest country, and say Geshev failed to bring corrupt high-level officials to account.

Read more: Thousands protest in Bulgaria against alleged government corruption

Wednesday's rally took place on the day of the first sitting of parliament's autumn session after its summer recess, and protesters called it the "Grand National Uprising" because of plans for the proposed constitutional amendments.

Inside parliament, President Rumen Radev renewed calls for the resignation of Borissov's center-right government and appealed to deputies to dismiss its plans.

"There is no way of emerging from this dead-end street other than peacefully, with the usual political act of resigning when confidence is lost," Radev said.

"It was not the lack of a new constitution that brought the people on the streets, but the lack of morality in the leadership, the erosion of statehood and the corruption," he added.

Borissov's GERB party and its coalition partners, the right-wing United Patriots alliance, put forward a draft new constitution in the national assembly on 17 August, in an effort to quell protests that had entered their 40th consecutive day.

Borissov also pledged to resign if parliament approves his call for the election of a grand national assembly tasked with voting on the constitution.

The proposed constitutional amendments would cut the term of office of the chief prosecutor from seven to five years and overhaul Bulgaria's top judicial body by splitting it into two councils that would deal separately with prosecutors and judges. It would also cut the number of parliamentary deputies from 240 to 120.

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In a video address on 14 August, Borissov said the proposals would increase confidence in political representation, strengthen the independence of the judiciary, make prosecutors and judges self-governing communities "and perhaps most importantly, strengthen control of the activities of the prosecutor-general."

But the amendments have been widely criticized by protesters and opposition figures for deepening the status quo rather than offering any kind of reform, and were dismissed as a ploy aimed at keeping Borissov in office longer.

Borissov needs the support of two-thirds of Bulgaria's 240 deputies to endorse the amendments, but it is not clear whether he will be able to muster it, and debates in parliament on whether to adopt a new constitution can take months.

The European Commission had placed Bulgaria under a corruption monitoring scheme when it joined the EU in 2007 over concerns it had significant problems with corruption and organized crime, and that the judiciary was not strong enough to ensure the rule of law – until last year, when the Commission proposed removing it from the program. It was the only EU member state, along with Romania, to be subject to such monitoring. 

Bulgaria also ranks as the EU's most corrupt member according to Transparency International and is yet to jail a senior official on corruption charges.