Bulgarian President Rumen Radev on Friday accused neighbouring Turkey of "interference" in the country's March 26 general elections and warned his government would not accept it.
"Turkey is our neighbour, friend and partner and we insist on developing good neighbourly relations," Radev told public BNT television on Friday.
"But Turkey's interference in our elections is a fact, and this interference is inadmissible."
The statement came after over a week of escalating tensions between the two countries.
Bulgaria is angered at Turkey's open support for Dost, a party for the ethnic Turkish minority, which is running in the general elections for the first time.
The government in Sofia summoned Turkey's ambassador and recalled its own envoy from Turkey for consultations on Thursday.
Radev called for "more calm and de-escalation of emotions" but also warned Bulgaria was vigilant.
"Bulgaria's institutions and relevant services are actively working on eliminating all interference in our electoral process and our internal affairs," he said.
Separately, Bulgaria's intelligence service, DANS, on Friday said that one Turkish national had been expelled and two more banned from entering or residing in the country.
One of the men was inciting anti-Bulgarian feelings in regions with a mixed Bulgarian and Turkish population, it said.
The long-time leader of the main MRF Turkish minority party in Bulgaria, Ahmed Dogan, lashed out against President Recep Tayyip Erdogan on Friday, calling his April 16 referendum on expanding the president's powers "madness."
Bulgaria is home to a 700,000-strong ethnic Turkish minority, a legacy of the Ottoman empire.
An additional 200,000 ethnic Turks with Bulgarian passports, who left Bulgaria during the Communist era, reside in Turkey and a third of them regularly take part in Bulgaria's elections.
Relations between ethnic Bulgarians and Turks have been peaceful, but many in the Turkish minority have bitter memories of assimilation policies from the Communist era, when the authorities forced them to adopt Slavic names.
Tensions between the two neighbouring countries also come at a time of a wider row between Ankara and the European Union ahead of the Turkish referendum, with a number of countries preventing Turkish ministers from attending referendum rallies.
Turkey has blasted German and Dutch politicians as "Nazis" and threatened to scupper a 2016 deal with the EU to brake the flow of migrants entering the bloc.
The row could be a major problem for Bulgaria, the EU's poorest country, since it shares a 270-kilometre (165-mile) border with its southeastern neighbour, Radev said Friday.
"Escalation of the tensions along the EU-Turkey axis will rebound most powerfully on Bulgaria, because we are on the front line," he warned, urging the EU to find a solution that guarantees the security of all member-states.