ISTANBUL — A village leader shoves four voting slips into a ballot box. An unknown arm marks three slips with a “yes” vote. An unknown hand adds five more. An election official validates a pile of voting slips — hours after they were meant to be validated.

These are four of the scenes captured in unverified videos that have helped stoke accusations of voting fraud in polling stations across Turkey during Sunday’s referendum to expand the powers of President Recep Tayyip Erdogan.

Mr. Erdogan’s “yes” campaign has claimed victory by a small margin — 51.4 percent to 48.6 — in a vote that further insulates the president from scrutiny and tightens his grip on one of the most influential countries in the region.

But while Mr. Erdogan has turned his claimed victory into a political reality, the legitimacy of his win is still in question. Opposition parties say the vote was rigged. The main opposition party formally asked Turkey’s electoral commission Tuesday afternoon to reassess the contents of multiple ballot boxes and — in a separate appeal — to annul the entire poll result. And two major international observation missions have a list of concerns over irregularities during the campaign and on the day of the vote.