Bulgaria's prime minister was hospitalised again Sunday as tens of thousands of people nationwide, unappeased by the government's recent resignation, staged angry protests against poverty, cronyism and corruption.

Around 7,000 demonstrators, according to an AFP estimate, blocked traffic on several key boulevards in the capital Sofia for hours, waving white-green-and-red Bulgarian flags and shouting "Mafia!"

Metal fences and a heavy police presence prevented both protesters and ordinary Bulgarians from attending an official flag-raising ceremony for liberty day celebrations commemorating the 1878 end of Ottoman occupation.

Over 20,000 demonstrators also gathered in the Black Sea city of Varna, a main centre of protests that have rocked the country over the past 20 days, calling for the resignation of mayor Kiril Yordanov, local media reported.

Meanwhile, Prime Minister Boyko Borisov, a former bodyguard and tough-guy police chief, was hospitalised with severe hypertension and weakness for a second time this week, his blood pressure hitting a high 180/100.

"Mr. Borisov was hospitalised again around 1:00 pm (1100 GMT) Sunday... We are monitoring him, his blood pressure is still high but we hope to normalise it," hospital chief Lyubomir Spasov said.

Smaller rallies were also held in about a dozen other cities, gathering between several hundred and several thousand people, reports said.

Over 3,000 disillusioned Bulgarians also joined a protest march of the ultra-nationalist Ataka party in Sofia, shouting "Let's get Bulgaria back!"

Bulgaria has been rocked for three weeks by sometimes violent demonstrations over high electricity prices that snowballed into public anger against wider issues in the European Union's poorest member state.

The rallies forced the surprise resignation on February 20 of once-popular Borisov, five months before his government's term was due to expire in July.

President Rosen Plevneliev is due to appoint a caretaker cabinet to govern the crisis-hit country ahead of early elections on May 12.

Political analysts however warned that the vote could fail to solve the political stalemate by returning a highly fragmented new parliament and making the formation of a new government extremely difficult.

The latest opinion polls showed Borisov's GERB and the Socialists neck-and-neck, both garnering about 22 percent support.

In a bid to calm down the protests, Bulgaria's energy regulator announced this week a cut in electricity prices by an average of 7.0 percent from next Tuesday.