Bulgarian energy watchdog is considering the option of revoking the license of the local unit of Czech power utility CEZ to operate the country's national grid.

This was announced by Economy and Energy Minister Delyan Dobrev at a press conference on Sunday as tens of thousands of Bulgarians protested in more than 20 cities against high electricity bills.

In the capital Sofia casualties and arrests were reported as protestors clashed with police who tried to cut short the rally heading for the headquarters of power utility CEZ.

Protesters hurled rocks and glass bottles at the nearby building of right-wing party UDF and the security forces, who responded by pushing them away and dispersing the rally.

CEZ supplies power to over 2 million households and companies in western Bulgaria, including the capital Sofia.

CEZ Bulgaria may be stripped of its national grid operator's license over irregularities in signing contracts with subcontractors, the minister specified.

It was not immediately clear whether the alleged irregularities can provide legal grounds for withdrawing the company's license.

The minister accused the company of granting a whopping 80% of its orders to companies without signing contracts or holding contests.

According to data, which he cited on Sunday, in 2011 CEZ Bulgaria assigned orders to companies, linked to the mother company, worth BGN 35 M.

Earlier on Sunday the mass protests against high electricity bills that swept Bulgaria's capital Sofia forced Czech power utility CEZ to offer compensations to its disgruntled clients.

"We fully understand the anger of the people.  If needed, we will pay compensations to our clients," Petr Baran, Vice Chairman of the Management Board and Chief Operational Officer of CEZ Bulgaria, told Kanal 3 TV Channel.

"We serve more than 2 million clients. Mistakes happen. Each complaint will be carefully considered and checked. If there are mistakes, corrections will be made," he added.