BELGRADE, Serbia (AP) — Thousands of supporters of Serbia's populist leader, Aleksandar Vucic, packed the largest indoor arena in Belgrade on Saturday for the de facto launch of his campaign for prime minister that will culminate in a pivotal election set for Oct. 25.

The early election, which polls suggest could be tight between Vucic's party and a group of candidates fielded by a student-led protest movement, was called after Vucic resigned as president on Sept. 27, cutting short his second term by more than seven months.

Vucic, who is constitutionally prohibited from running for a third term as president, declared upon resigning that he would lead his populist Serbian Progressive Party in the parliamentary election. If his party wins a majority of seats, the parliament is expected to elect Vucic as prime minister.

Dozens of buses from across Serbia clogged streets around Belgrade Arena on Saturday as supporters arrived at the rally in the capital carrying Serbian flags and banners bearing Vucic’s picture.

The event, televised by dozens of channels across Serbia, capped a week of Vucic touring towns and villages in an apparent bid to shore up his popularity that's been shaken after almost two years of sustained anti-government protests.

The youth-backed movement that led the protests has demanded accountability for the Nov. 1, 2024 train station canopy collapse that killed 16 people in the northern city of Novi Sad.

The movement's struggle for justice over the tragedy and against corruption has grown into a nationwide political force whose electoral arm, the Student List, will attempt to oust Vucic’s government after 14 years in power.

Vucic himself has been either prime minister or president for the past 12 years.

The Student List includes university professors, teachers, experts and others from across the political spectrum. Yet Vucic has accused those behind the movement of seeking to “bring Serbia to a halt,” and claimed, without providing evidence, that it's backed by unspecified foreign agents.

On Saturday, Vucic told his supporters: “Absent an intervention of some (foreign intelligence) service, we will defeat them. And they know that.”

The election is taking place amid heightened political tensions in Serbia, with reports of incidents and physical altercations at campaign events. International observers have described previous elections in Serbia as marred by multiple irregularities and violence.

While formally a candidate for European Union membership, Serbia’s accession process has stalled under Vucic, who maintains close ties with Russia and China.

European institutions have warned of democratic backsliding in Serbia in successive reports tracking the progress of candidate countries. Rights and media freedom groups have separately warned of police violence, arbitrary detentions, unauthorized surveillance and threats against journalists.