In a hundred days of sweeping and at times violent protests, thousands of Bulgarians have tightened the screws on the political elite -- but this, they say, is only the beginning.

Thousands of Bulgarians protested in the capital Sofia for an eighth consecutive day Thursday to demand the end of a government they accuse of corruption and favouritism towards oligarchs.

Bulgaria's defence minister has hit out at "unreasonable" Greek plans to build a camp near their border to shelter asylum seekers amid a migration surge from Turkey.

Bulgaria introduced mandatory voting on Thursday, with the government hoping to boost low election turnout and battle rampant vote-buying in one of the European Union's most corrupt countries.

Under the new measures approved by parliament, those failing to cast ballots will be deleted from voting registers and will need to register again if they want to take part in subsequent elections.

A citizen's arrest of three illegal migrants sparked controversy in Bulgaria on Monday, with rights groups protesting while the government refrained from condemning the practice.

Amateur video footage broadcast on several Bulgarian televisions and in social media showed three men lying on the ground, their hands tied behind their backs as someone shouts: "Go back to Turkey!"

Bulgaria is bracing for increased migrant pressure along its southern border with Greece and is ready to install a razor-wire barrier at any time, Prime Minister Boyko Borisov said Friday.

"The major threat that I see at the moment is from the Greek border, which is very long and unprotected. We are worried that in the last several months the Greek government has not taken any measures," Borisov told parliament, adding that the conditions in the Greek refugee shelters were "horrible."

A Bulgarian border police patrol on Wednesday found the dead bodies of two men -- presumed to be migrants -- in a region close to the country's border with Turkey, authorities said.

Border police spokeswoman Lora Lyubenova told AFP that the men, found in the Malko Tarnovo region, had no documents and so could not be identified.

Thirteen men from Bulgaria's Roma minority practicing an ultraconservative form of Islam went on trial Thursday charged with helping people join the Islamic State group in Syria and propagating extremist ideology.

The accused form part of a Wahhabi community several hundred strong in several cities set up by self-proclaimed Roma imam Ahmed Moussa Ahmed, 40, who was already sentenced to two years in jail in 2015 for inciting hatred on religious grounds.

Three Syrians living in Germany have been arrested in Bulgaria on their way to join Islamic State fighters back home and are facing terrorism charges, prosecutors in Sofia said Tuesday.

It is the first such case in Bulgaria since it introduced new legislation last year criminalizing the intent of joining terrorist groups.

Bulgarian prosecutors said Tuesday they had charged five border police with being part of an organized crime group and accepting bribes from migrant smugglers at the country's southeastern border with Turkey.

The suspects, including three high-ranking officials, were arrested on Monday.