European election monitors are criticizing a Turkish decision to deny entry to two members for alleged bias against Turkey.

Peter Osusky, head of the delegation from the Organization for Security and Co-operation in Europe Parliamentary Assembly told The Associated Press on Sunday that all observers “are strongly adhering to so-called code of conduct” regardless of their political opinions.

Turkey denied entry to Andrej Hunko of Germany’s Left Party and Jabar Amin of Sweden’s small Environment Party “based on their publicly expressed political opinion.”

The OSCE’s Office for Democratic Institutions and Human Rights decided to deploy 22 long-term and 350 short-term observers for Turkey’s presidential and parliamentary elections Sunday. Opposition leaders fear recent changes to electoral procedures could lead to voting fraud. The BBC's Turkish correspondent tweeted a video on Sunday which allegedly showed ballot stuffing at a polling station in Urfa - the video was not independently verified.

This week the Turkish state-run news agency carried a story suggesting that the OSCE observer mission was biased against Turkey. Ignacio Sanchez Amor, leader of the short-term OSCE observer mission, called the story “a complete fabrication.”

Two main candidates posing a tough challenge to President Recep Tayyip Erdogan in Turkey’s presidential and parliamentary elections have cast their votes and vowed vigilance amid fears of possible fraud.

Erdogan and his ruling party are the seen as the front runners in the dual polls but for the first time in his 15-year-rule, the Turkish leader is facing a united and more energized opposition.

Rallies by Muharrem Ince of the secular opposition Republican People’s Party, or CHP, have drawn massive crowds, while Meral Aksener of the newly-formed nationalist Good Party is trying to attract conservative votes away from Erdogan’s ruling party.

Both leaders on Sunday alluded to fears of vote-rigging.

Ince voted in his home town of Yalova in northwest Turkey. Aksener told reporters in Istanbul: “I hope these elections are beneficial and truly reflect the free will of the voters.”