Bulgaria's former prime minister, Boyko Borisov, was detained late on March 17 as part of a police operation linked to probes by the EU prosecutor's office, the Interior Ministry said.
Former Finance Minister Vladislav Goranov, the former chief of the parliamentary budgetary commission, Menda Stoyanova, and Borisov's media adviser, Sevdalina Arnaudova, were also detained, the ministry said in a statement.
"A large-scale operation is under way in connection with 120 cases of the European Prosecutor's Office in Bulgaria. Searches and seizures are carried out at many addresses. Boyko Borisov, Vladislav Goranov, Menda Stoyanova, and Sevdalina Arnaudova are currently detained," the ministry said.
Borisov, 62, is currently the leader of opposition center-right GERB party.
The ministry did not provide details, but public BNT television said the cases concerned "misuse of EU aid."
Searches and seizures continued at many addresses across the country.
"Nobody is above the law!" Bulgaria's new Prime Minister Kiril Petkov posted on his official Facebook page late on March 17.
Petkov's newly formed anti-corruption party won general elections in Bulgaria last year on a platform of "zero tolerance for corruption."
The operation comes after a visit to Sofia on March 16-17 by European Chief Prosecutor Laura Koevesi. 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," Koevesi said.
"This is why we exist. This is why we are here," Koevesi said, adding that European prosecutors have opened 120 investigations of fraud involving EU money related to public tenders, agricultural subsidies, construction, and coronavirus recovery funds.
Borisov has been accused of corruption several times over the years but has denied any wrongdoing.