The protests mark a further escalation of a spat between EU member Bulgaria and its neighbour Turkey.
The protests mark a further escalation of a spat between EU member Bulgaria and its neighbour Turkey over Ankara's open support for a new party representing Bulgaria's Turkish minority.
About 50 protesters gathered at Kapitan Andreevo, the main checkpoint, with banners reading "Bulgaria above everything!" and "No to Turkish interference!" as loudspeakers blared patriotic songs, an AFP photographer said.
"We will remain here day and night to preserve the sovereignty of our homeland," protest organiser Valeri Simeonov from the United Patriots nationalist coalition told AFP from the checkpoint.
Bulgaria is home to a 700,000-strong Muslim minority, most of them ethnic Turks, while at least 200,000 ethnic Turks with Bulgarian passports live in Turkey.
Traditionally they have voted for the centrist Movement for Rights and Freedoms (MDL) but the party has become increasingly critical of Turkey and its President Recep Tayyip Erdogan.
Last year a splinter group split off and formed a new party, Dost, which openly backs Ankara.
Turkish officials including Turkey's ambassador in Bulgaria have openly voiced their support for it.
Sofia has complained about Turkish meddling and in the latest angry exchange on Thursday, Erdogan accused Bulgaria of putting "serious pressure" on its Turks ahead of the vote.
Bulgarian President Rumen Radev said that "Bulgaria neither gives nor accepts lessons in democracy, especially from countries that do not respect the rule of law."
Between 60,000 and 90,000 of the Bulgarian citizens in Turkey regularly vote in Bulgarian elections but new rules slashed the number of polling stations in Turkey to 35 from more than 100.
BNR Bulgarian public radio reported from the northwestern Turkish city of Bursa that Turks who wanted to travel to Bulgaria for the vote were offered free transport by Turkey.
The spat has boosted support for Bulgaria's nationalists, with polls suggesting the United Patriots will come third in Sunday's election behind the Socialists and the centre-right.
Turkey and other EU countries are embroiled in a wider spat ahead of an April 16 referendum on creating an executive presidency that critics say will give Erdogan too much power.