ISTANBUL — Violence has marred the run-up to Sunday's elections in Turkey, in which President Recep Tayyip Erdogan is seeking another five-year term amid a crackdown on dissent following the 2016 failed coup.

At least 87 opposition supporters suffered beatings or were injured in incidents from April 26 through June 20, according to a report by the Human Rights Association, a Turkish NGO. The victims include four supporters of the Good Party who were stabbed last month at a campaign booth by members of a rival opposition party. Three members of Erdogan’s Justice and Development Party (AKP) have also been injured during campaigning.

The Human Rights Association said other tactics used against activists during the campaign included threats, bans on rallies or “interventions” such as preventing supporters from handing out flyers. All but two of the 132 incidents cataloged in the report were aimed at opposition parties.

The violence has worsened recently, with four people killed last week in a clash during a campaign visit by an AKP candidate in the predominantly Kurdish southeast.

Turkey's state news agency said local store owners attacked the AKP members, but the BBC cited opposition sources as saying the candidate’s bodyguard opened fire. NBC News could not independently verify either report.

A reflection of a poster showing Turkish President Recep Tayyip Erdogan in Istanbul. Emrah Gurel / AP

An election center run by the main opposition Republican People's Party (CHP) was also ransacked last week. The CHP blamed young members of the AKP and alleged that "mafia methods" were being used on the campaign trail.

With the majority of media outlets under the control of Erdogan and his allies, all opposition candidates are struggling to get their voices heard.

The Turkish president's response to the failed 2016 coup was to embark on a purge of opponents he claimed were linked to Fethullah Gulen, a cleric who lives in self-imposed exile in Pennsylania. Erodgan accuses Gulen of masterminding the coup. Gulen denies the allegation.

Around 160,000 people have been detained as part of the post-coup crackdown, according to the U.N. Human Rights Office. Academics, opposition politicians and journalists are among those behind bars. The country remains under a state of emergency.

Voters will choose a Parliament as well as a president on Sunday, and polls suggest both elections will be closer than expected. Around 55 million people have registered to vote in Turkey, along with 3 million overseas.

If no presidential candidate gains 51 percent, a second round of voting will be held. Erdogan has ruled Turkey since 2002, first as prime minister and now as president.

Howard Eissenstat, a professor of Middle East history with a focus on Turkey at St. Lawrence University in Canton, New York, warned that violence "would almost certainly intensify" in the event of a second round.

The Washington-based Bipartisan Policy Center warned a fraudulent victory could lead to “widespread opposition protests [which] would be met with sustained violence from either police or organized groups of AKP supporters.”

American citizens attending a recent U.S. government function in Istanbul were warned of possible street-level violence following this weekend's elections, but officials would not provide an on-the-record comment to NBC News.

Britain's Foreign and Commonwealth Office has also warned that the elections may result in demonstrations, and urged its citizens to stay away from large gatherings.

An Organization for Security and Cooperation report highlighted that the pro-Kurdish People's Democratic Party (HDP) canceled a rally scheduled for the capital of Ankara on June 9 after police said they could not guarantee security as a result of an AKP event that was to be held nearby.

Eissenstat said Erdogan has politicized the police and military over the past 15 years. The AKP did not respond to a request for comment.

Mert Eryigit, 15, a high school student, says he was beaten by police after attending a pre-election rally demanding education reforms in the Kadikoy district of Istanbul, considered an opposition stronghold.

Eryigit says he believes his blood-stained vest should be seen as a warning to opponents of Erdogan.

"They want to show what would happen if Erdogan didn’t win … what would happen if people went on the streets after the election,” he said.

Erdogan called snap presidential and parliamentary elections a year and a half ahead of schedule. They will usher in a new executive presidential system that increases the powers of the president.

Supporters of imprisoned Selahattin Demirtas, the HDP's presidential candidate, in Istanbul on Sunday. Sedat Suna / EPA

The five candidates running against Erdogan include Selahattin Demirtas, a former human-rights lawyer who is behind bars fighting terrorism-related charges. Demirtas is the former co-chairman of the HDP and one of the country's best-known politicians.

He faces a 142-year sentence if convicted. Demirtas, who denies the accusations, was allowed to run in the presidential race because he has yet to be convicted. It wasn't clear, however, if or how he would be able to take up office in the unlikely event that he is elected.

Muharrem Ince, a 54-year-old former physics teacher, is seen as the top contender to Erdogan in the race to be president. However, his CHP has struggled to win support beyond its core base of secular-minded voters. In the last parliamentary election in November 2015, it took 25.3 percent of the vote.

Enes Bayrakli, director of European studies with the pro-government SETA think tank, said that violence during the campaign has been limited.

"This is not really a widespread phenomenon, so these are minor issues,” he added.