ISTANBUL—Voters are signing up by the tens of thousands to safeguard ballots in Sunday’s parliamentary election, the most uncertain in a decade, highlighting eroding faith that the vote will be conducted fairly.

More than 50,000 volunteers have joined the civilian initiative Vote and Beyond to monitor voting and vote-counting in 41 of Turkey’s 81 provinces, said Sercan Celebi, co-founder of the organization, with many more volunteers expected. The observers will oversee more than 60% of total votes cast, he said.

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