Ekrem Imamoglu of the main opposition party was speaking atop campaign bus when a large group began throwing rocks.

More than a dozen people have been arrested over violence against opposition supporters at an election rally in Turkey’s eastern city of Erzurum, according to the justice minister.

Istanbul Mayor Ekrem Imamoglu of the main opposition Republican People’s Party (CHP) was speaking from the top of a campaign bus on Sunday when a large group of roughly 200 protesters began throwing rocks.

Imamoglu was campaigning on behalf of CHP leader and presidential candidate Kemal Kilicdaroglu, the main opponent to President Recep Tayyip Erdogan, days before Sunday’s presidential and parliamentary elections.

He maintained that police and pro-Erdogan officials in Erzurum allowed the attack to go ahead.

“There are citizens here who are injured, and you police are just standing by,” Imamoglu is heard saying in footage of the incident. “Erzurum governor, Erzurum mayor, I will be filing a criminal complaint against you.”

The video later shows Imamoglu being taken inside by his entourage, the bus driving off and police later dispersing the group with water cannon.

Seven people were injured in the incident, according to Erzurum Governor Okay Memis.

Speaking on the news channel A Haber on Monday, Justice Minister Bekir Bozdag said an investigation was being conducted and 13 people had been arrested so far.

Kilicdaroglu posted a video on social media following Sunday’s incident in which he accused the people responsible of being a “militarist coalition” who seeks to “scare people away from the ballot box”.

Speaking on a television program later in the evening, Interior Minister Suleyman Soylu claimed the incident was “theatre”.

“They provoke such incidents. Then they claim, ‘They did this to us. They did that to us,’” he said. “… They had put forward a speech that was meant to agitate, to provoke the crowd.”

Erdogan’s biggest challenge

Sunday’s elections are seen as among the most consequential in Turkey’s modern history. Erdogan is facing the biggest political challenge of his two-decade rule.

Surveys carried out since devastating earthquakes in southern Turkey on February 6 show Kilicdaroglu ahead of Erdogan.

If no candidate secures more than half the votes in the first round of voting, a May 28 run-off will be held between the two leading candidates.

Most polls for the parliamentary elections show the main opposition alliance ahead of the People’s Alliance, which includes Erdogan’s Islamist-rooted AK Party, the nationalist MHP, the right-wing Great Unity Party and the New Welfare Party.

The opposition Nation Alliance includes Kilicdaroglu’s main opposition CHP, the centre-right IYI Party, the Islamist Felicity Party, the Democrat Party and two parties founded by former Erdogan allies, the Deva and Future parties.

Parliament’s third-largest party, the pro-Kurdish Peoples’ Democratic Party, will run for parliament under the Green Left Party banner to circumvent its potential closure. It has formed the Labour and Freedom Alliance with several other left-wing parties.

With votes already being cast at polling stations outside of the country, alliances or parties that run alone must secure at least 7 percent of the total votes cast to win seats in parliament.