Bulgaria's former prime minister Kiril Petkov was charged with abuse of power on Friday, for ordering the late-night arrest of his predecessor and two of his closest aides.

Three-time premier Boyko Borisov, his former finance minister and press service chief were detained in March 2022 as part of a probe into suspected extortion and possible misuse of EU funds.

All three were released after a day without charge, as prosecutors found procedural flaws with the police probe and a lack of evidence to indict them.

Petkov was summoned for questioning in Sofia on Friday and denied any wrongdoing, telling reporters as he emerged from the prosecutor's office in Sofia: "I have been indicted."

Prosecutor Angel Kanev said that Petkov, co-leader of the reformist PP party, had overstepped his powers as prime minister in 2022 and "ordered, without having legal grounds, the arrest of certain people".

He had also "ordered police officers to carry out searches contrary to the law, in the dark of day, without the permission of a court," Kanev added.

Conservative GERB party leader Borisov has denied any fault and has repeatedly accused Petkov of ordering his arrest to humiliate him and as part of his efforts to promote his anti-graft cabinet.

Bulgaria, which joined the European Union in 2007, remains one of the bloc's most graft-ridden member states, according to Transparency International's Corruption Perception Index.

Massive anti-corruption street protests in 2020 had toppled Borisov after close to a decade in power.

But the ambitious anti-graft cabinet that followed led by Petkov lost a coalition partner and fell after a no-confidence motion filed by Borisov's GERB party after only seven months in power.

Since 2021 the country has been plunged into its deepest political crisis since the end of communism, holding seven successive general elections without managing to form a stable government.

GERB won the latest general vote in October but its ongoing talks on forming a possible three- or four-party coalition are still far from an agreement that could give his proposed cabinet a majority in a badly-fragmented parliament.