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Protesters chanting "It will be the same every day until we win", paralysed city centres and demanded the resignation of the cabinet and the re-nationalisation of power distributors.
Demonstrators hurled eggs and bottles, burned their electricity bills and attacked power firms' offices in cities across the country.
Electricity prices are politically sensitive in the European Union's poorest member since power bills bite off a big chunk of monthly incomes, especially during the winter.
The ruling centre-right GERB party has seen its popularity flag since last year with voters disgruntled by low incomes and high bills.
Responding to Sunday's protests, Economy Minister Delyan Dobrev told reporters: "In the coming days, we will take a final decision on whether there are grounds for revoking the licenses of the power distributors."
Bulgaria's power distribution market is divided into three regions, controlled by Czech firms CEZ and Energo-Pro and Austria's EVN.
"We fully understand the anger of the people," CEZ Bulgaria's vice-chairman Petr Baran told local media. "If needed, we will pay compensations to our clients."
Many protesters said they had been overcharged in their December bills and an avalanche of complaints has been lodged with the power distributors.
"Each complaint will be carefully considered and checked," Baran added. "If there are mistakes, corrections will be made."
Sunday saw the biggest demonstrations in eight days of protests. More than 2,000 people blocked the highway to Greece near the southern town of Dupnitsa and in the capital Sofia protesters blocked traffic on the famous Eagles' Bridge.
"We cannot stand it anymore," said Penka Slavova, a pensioner. "My pension is 155 levs ($110) and my December bill was 175 levs. What should I do?"
In one of the biggest rallies, more than 10,000 people marched in Varna on the Danube River, where the national flag had been lowered at the municipality in support of the protests.
Local media reported that protesters clashed with police when trying to reach the Sofia headquarters of CEZ and four demonstrators were arrested.
LOW LIVING STANDARDS
Despite enjoying relative economic stability since the global financial crisis erupted, Bulgarians face low living standards compared to other EU members. Monthly pay averages 400 euros, just a fraction of the EU norm.
The government has said it will look into the issue of rising electricity bills, but has ruled out the re-nationalisation of power firms.
Support for Prime Minister Boiko Borisov's GERB party fell to 22.6 percent in February, down from 23.8 percent in January due to delayed reforms, low incomes and a lack of action to root out corruption. Unemployment is at a 10-month high.
Backing for the opposition Socialists, who said they would consider re-nationalisation of the power distributors if they win the parliamentary elections on July 7, rose to 22.1 percent.
"We should remember this date - February 17," said political scientist Evgeniy Daynov of Sofia's New Bulgarian University.
"This is when the ruling GERB party died. This is a protest against the oligarchic model of GERB rule, which is nothing more than the plundering of the state to benefit the oligarchs."
Bulgaria has long been criticised for failing to liberalise its highly monopolised electricity and gas distribution markets in line with the EU rules.
In January, the European Commission referred the Balkan state, Estonia and Britain to the European Court of Justice - the highest EU court - after the three countries had only partially transposed EU energy market directives.