Turkish voters have begun casting their ballots in presidential and parliamentary elections that pose the biggest challenge to Tayyip Erdogan.

Voters began gathering at polling centers early on Sunday to cast their ballots in the elections that complete Turkey's transition to a new executive presidential system that was approved in a controversial referendum last year.

Turkish presidential candidate Muharrem Ince, the main challenger to President Recep Tayyip Erdogan in Turkey's June 24 presidential election, held a campaign rally in Istanbul on the eve of the election, in front of thousands of supporters.

More than 56 million people are eligible to vote across the country. Over 3 million Turkish expatriates cast their ballots in the elections last month.

Turkey is gearing up for early presidential and parliamentary elections. Incumbent President Recep Tayyip Erdogan is facing a tough challenge from his main rival, Muharrem Inje.

Many political experts believe Inje is capable of dragging the race into a run-off. Four other candidates are also running for presidency.

The winner will get new sweeping powers under a constitutional shake-up approved in a referendum last year.

Sunday’s elections were originally scheduled for late 2019. Erdogan brought the elections forward.