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A decade-old arrest of Councilman Jumaane Williams has surfaced in the final stretch of the New York City public advocate’s race, roiling what had already been an unpredictable contest as his rivals demanded more information just before the polls were set to open on Tuesday.

Mr. Williams, 42, on Monday acknowledged that he had been arrested a decade ago on harassment and criminal mischief charges and spent a night in custody after what he called a “verbal disagreement” with his girlfriend at the time. Charges were dropped and the arrest record itself is sealed.

Mr. Williams is seen as a leading candidate in the public advocate contest after he finished a close second in his insurgent run for lieutenant governor last year against the Democratic incumbent, Kathy Hochul.

The field for the special election features 17 candidates, and several female rivals used the last-minute revelation of the arrest to raise questions about Mr. Williams’s fitness for the position. Another City Council member, an ally of Mr. Williams, called for an investigation into who leaked what had been a sealed arrest record.