Bulgarian nationalists blocked border checkpoints with Turkey on Friday to stop buses bringing Bulgarian ethnic Turks to vote in Sunday's election amid growing tension between the two neighbours.
Sofia has accused Ankara of interference in its election on behalf of the DOST party that represents Bulgarian Turks, the country's largest ethnic minority. Polls suggest DOST will fall short of the four-percent vote needed to enter parliament.
With slogans reading "Hands off Bulgaria" and "No to electoral tourism", about 100 supporters of the nationalist coalition United Patriots blocked the main checkpoint at Kapitan Andreevo-Kapikule and vowed to stay until the end of the vote.
Tensions have risen also between Ankara and the broader European Union since Germany and the Netherlands blocked Turkish ministers campaigning among expatriate Turks in their countries for a referendum that would give Turkish President Tayyip Erdogan broader executive powers.
The United Patriots said more than 1,000 buses with voters from Turkey were expected to cross the border - a movement they say is organised and paid for by the Turkish state to increase support for the DOST party.
"It is not normal for a foreign country to interfere in the election of another sovereign state," Krasimir Karakachanov, co-leader of the United Patriots, told Reuters. "Turkey is now trying to provide voters for DOST, a pro-Turkish party, because it knows that the support for this party in Bulgaria is very low."
Erdogan accused Bulgaria on Thursday of putting pressure on Bulgarian ethnic Turks living and working in Turkey and flouting democratic principles.
Bulgarian security services have expelled two Turkish citizens and banned another three from entering the country, saying they were inciting anti-Bulgarian sentiment in areas of mixed population.
The nationalists, whose support has increased with the inflow of mainly Muslin migrants to Europe, are seen coming third in the snap polls this Sunday and are expected to play a kingmaker role in forming a new government.
European Union member Bulgaria stands out in the Balkans for its significant Muslim minority, some 12 percent among 7.2 million mainly Orthodox Christians - a legacy of almost 500 years of Ottoman Turkish rule that ended in the late 19th century.
The resistance to the Muslim Ottomans is a core element of Bulgarian national identity. About 400,000 Bulgarian nationals live in Turkey, most of them Bulgarian Turks descended from Ottoman-era Turkish settlers in the Balkans.