At least six people were arrested by Turkish authorities late on Saturday night for allegedly insulting President Recep Tayyip Erdogan on social media ahead of a campaign rally by his main opponent in Sunday's high-stakes presidential and parliamentary election.

State-run Anadolu news agency said the six were detained after videos posted on social media reportedly showed them shouting expletives against Erdogan. The news agency said they were later charged with "insulting state elders".

The agency said police were searching for other suspects.

Insulting the president is a crime punishable by up to four years in prison. Erdogan has filed close to 2,000 lawsuits against people, including school children, for insulting him.

As a goodwill gesture, he dropped the cases following a failed military coup in 2016. But many more cases have been filed since then.

Voters are flocking to polling centres today to cast ballots in an election that will complete Turkey's transition to a new executive presidential system, a move approved in a controversial referendum last year.

Erdogan, 64, is seeking re-election for a new five-year term, and his ruling Justice and Development Party, or AKP, is hoping to retain its majority in parliament.

Erdogan has overseen historic change in Turkey since his Islamic-rooted ruling party first came to power in 2002 after years of secular domination. But critics accuse the Turkish strongman of trampling on civil liberties and displaying autocratic behaviour.

The president is facing a more robust and united opposition this time, one that has vowed to return Turkey to a parliamentary democracy with strong checks and balances and decried what it calls Erdogan's "one-man rule".

Today's polls could either consolidate Erdogan's hold on power or curtail his vast political ambitions.