Bulgaria headed for early election by mid-May

* Main political parties declined chance to form government

* Bulgaria to have interim government

* More protests over electricity prices planned

By Tsvetelia Tsolova

SOFIA, Feb 22 (Reuters) - Bulgaria's president will appoint a caretaker government ahead of a parliamentary election by mid-May after protests toppled the austerity-minded cabinet of Prime Minister Boiko Borisov, the president said on Friday.

The resignation of Borisov's rightist government has failed to quell public anger over high utility bills, and more protests are planned on Friday evening, as well as over the weekend.

President Rosen Plevneliev said major political parties - including Borisov's GERB and their rivals the Socialists - declined the chance to form a new government and the poll will be brought forward from its previously planned date in July.

"We are heading towards an interim government. We have agreed that the possible timeline for next elections will be the end of April until the middle of May," Plevneliev told reporters after consultation with political parties.

The president will now have to appoint a caretaker administration, which he said would focus on ensuring free elections, possibly next week.

Growing public frustration at the government's failure to boost living standards in the EU's poorest member state boiled over into bloody protests this month.

Many in the Black Sea state of 7.3 million are also angered by Borisov's failure to make good on his 2009 election pledge to stamp out endemic corruption and reform inefficient healthcare and education systems.