Thousands of South Koreans were demanding a rerun of local elections as they protested into the wee hours of Sunday over ballot paper shortages that disrupted this week's vote.

Wednesday's election was the first nationwide vote since President Lee Jae Myung took office following conservative Yoon Suk Yeol's ouster over his brief martial law declaration in late 2024.

Lee's ruling liberal Democratic Party won most races in the elections for mayors, local government officials and assembly members, but failed to flip the critical Seoul mayoral seat.

National Election Commission (NEC) chairperson Rho Tae-ak resigned on Friday after public outrage over ballot paper shortages that the commission said affected 50 polling stations nationwide, including more than 30 in the capital.

On Saturday evening in Seoul, about 10,000 people gathered at the SK Olympic Handball Stadium, where ballots from the elections had been counted, Yonhap reported, citing an unofficial police estimate.

By 2:00 am Sunday, at least 1,000 protesters, many in their 20s and 30s, still remained at the site, waving large national flags and chanting, "Re-election, re-election," an AFP journalist saw.

Young volunteers handed out water, coffee and chocolates. Some demonstrators were accompanied by their pets; others sat on picnic mats, holding national flags and keeping watch through the night.

"Regardless of political affiliation, not being able to vote is an infringement on our rights in a free democracy," Seo Jin-hee, 31, told AFP, looking visibly emotional.

The NEC said ballot papers were printed for only 50 percent of eligible voters because there were large numbers of unused ballots in recent elections due to increased early voting.

Some voters are believed to have left without casting their ballots.

The commission's explanation "simply defies common sense to any ordinary citizen", said another demonstrator, Park Soun-wok, 29.

"It does not matter whether my preferred candidate wins or not. Regardless of all that, I believe this election must be rerun," he told AFP.

'Unacceptable by any standard'
Wednesday's election problems had triggered a 35-hour blockade of a polling station in Seoul, with protesters preventing authorities from removing two ballot boxes for counting.

Riot police dispersed the protesters on Friday morning, with videos on social media showing officers dragging an elderly man across the floor and clashing with demonstrators.

Despite NEC chairperson Rho's resignation later that day, protesters regrouped at the SK Olympic Handball Stadium, demanding a new election and a thorough investigation.

Claims from former president Yoon -- now jailed and on trial for insurrection -- that the NEC had ignored warnings about North Korean threats to voter data resonated with right-wing YouTubers and supporters, who spread unverified election fraud theories online and questioned the commission's credibility.

But Choi In-jae, 29, a volunteer sketching national flags at the demonstration, told AFP: "I believe (right-wing figures) make up only a very small part of the crowd here today."

"I myself came out simply because I wanted to help when people are standing up for their fundamental rights."

Analysts say the NEC, a constitutional body with limited external oversight, has long faced gaps in internal discipline and review mechanisms.

"This mishap by NEC was unacceptable by any standard," political commentator Park Sang-byung told AFP.

"When the integrity of elections is compromised, the foundation of democracy itself is at risk. Strict accountability and stronger checks and balances are desperately needed."