UPDATE 1 - Bulgaria's ruling GERB party to propose new Constitution - PM

Under the new Constitution the number of the members of Parliament would be cut by half - to 120, the number of the members of a Grand National Assembly would be reduced to 280 from 400 now, and several changes to the structure of the judiciary would be introduced, Borissov said in a statement.

"As soon as the National Assembly adopts a decision for elections for a Grand National Assembly to be held in accordance with Article 160 of the Constitution, I will step down as prime minister the same day," Borissov said.

"These are things that people have wanted and expected for years," Borissov noted. "In order to meet the expectations of the Bulgarian citizens, we propose a draft Constitution to protect the basic principles of our time," he added.

According to Borissov, as the current Constitution was adopted in a different time period, it does not provide sufficient tools to tackle modern-day challenges. Bulgaria's current Constitution was adopted in 1991, shortly after the fall of the communist regime in the country.

GERB insists that half of the members of the Grand National Assembly should be picked via a proportional electoral system, while the other half would be elected through majoritarian voting.

Borissov's statement comes as protests calling for the resignation of his coalition government and prosecutor general Ivan Geshev, and urging an overhaul of the judiciary entered their 37th day on Friday.