Bulgaria's former prime minister Boyko Borisov was detained late on Thursday alongside other members of his party in relation to EU prosecution probes focused on the misuse of EU aid funds.

"A major operation is underway in relation to 120 cases of the European Public Prosecutor's Office in Bulgaria... Boyko Borisov... has been detained," the interior ministry said in a statement.

Detained along with Bulgaria's three-time conservative premier were his ex-finance minister Vladislav Goranov, ex-parliamentary budget committee chairwoman Menda Stoyanova and former government press service chief Sevdalina Arnaudova. 

The ministry added that searches and seizures continued at many addresses across the country.

While it did not provide details of the investigation, public BNT television said the cases concerned "misuse of EU aid."

"Nobody is above the law!" Bulgaria's new Prime Minister Kiril Petkov posted on his official Facebook page on Thursday evening.

The operation follows European Chief Prosecutor Laura Kovesi's visit to Sofia this week, where she praised Petkov's "determination, leadership, and compelling vision on the fight against corruption."

"Now is the time for the relevant Bulgarian authorities to team up with us, including on particularly sensitive cases. This is why we exist, this is why we are here," Kovesi said.

She added that Bulgarians have sent a record number of reports about alleged criminal activity to to her office. 

European prosecutors have launched 120 investigations of fraud involving EU money related to public tenders, agricultural subsidies, construction, and coronavirus recovery funds, according to Kovesi. 

Petkov's newly formed anti-graft party unexpectedly won the general elections in Bulgaria in 2021 on a "zero tolerance for corruption" platform.

Borisov's nearly 10-year tenure was marked by multiple corruption allegations, including claims made directly against him. The former leader denies any wrongdoing.

In 2020, pictures were leaked to the media showing Borisov asleep with a handgun on his nightstand, while other pictures - which he said were doctored - allegedly show several bundles of 500-euro ($600) notes and gold bars packed into his bedside table.