JOVANA GEC BELGRADE, Serbia — Thousands of people lined the streets of Belgrade on Wednesday to greet marching students arriving to Serbia ‘s capital on the eve of their final rally ahead of a tense parliamentary election challenging authoritarian leader Aleksandar Vucic.

The early election on Sunday is seen as a showdown between the populist strongman and a youth movement that led almost two years of protests against his tight rule in the Balkan country.

Belgrade residents lit flares and cheered as they welcomed the students coming into the city from various directions after they had walked throughout the country toward Belgrade in the past several days.

Ilija Srdanovic, a physician who is a candidate on the Students’ List election ticket, told N1 television that “this is incredible, this energy is unstoppable.”Vucic on Wednesday held a rally in the southern city of Nis in front of thousands of his flag-waving supporters. Vucic in a speech expressed confidence that his right-wing Serbian Progressive Party will win the ballot on Sunday.

The former Serbian president is looking to return as prime minister at the vote after resigning the presidency in September so he could pursue the campaign. “There will be no more politics in schools and at faculties,” Vucic vowed at the rally.

He reiterated accusations that both the students and their professors worked under instructions from abroad to “breakdown” the country.

Sunday’s ballot is tense because both Vucic’s SNS and the Students’ List group are said to be polling close, which makes the outcome uncertain and fuels fears of incidents if both sides declare victory after polls close.

Authorities earlier on Wednesday detained six people allegedly suspected of planning unrest on election day. Police have made similar arrests ahead of major protests in the past which government critics say are aimed at intimidating protest-goers.

The student rally on Thursday in Belgrade is expected to draw thousands of people. It will be held at the central Slavija square where students staged their biggest gatherings during protests that rattled Vucic more than ever during his tenures as both prime minister and president in the past 12 years.

Exhausted but joyful, marching students on Wednesday evening walked a red carpet set up in central Belgrade for the welcome ceremony. Days-long student marches were aimed at reaching Serbia’s rural areas that are Vucic’s traditional strongholds.

Student demonstrations first erupted after a train station tragedy in Serbia’s north killed 16 people. Serbia’s youth demanded accountability for the deaths, blaming alleged corruption-fueled negligence during renovation work.

International election monitors and the European Union have expressed concern about the safety and fairness of the ballot following widespread irregularities in the past.

In a report published Wednesday, the Serbian Center for Research, Transparency and Accountability monitoring group cited “narrowing of freedoms and deepening of inequality” among election participants and voters. It said opposition activists are being monitored and questioned and their communications intercepted. The report cited a major discrepancy in the mainstream media coverage in favor of Vucic and noted over 100 violent incidents during the campaign.