South Korea on Thursday saw a record early voting on the first day of a two-day voting to elect a successor to former President Yoon Suk Yeol, who was ousted over his failed martial law bid, Yonhap News reported.

The polling stations in the capital Seoul have been crowded since the voting began at 6 am local time Thursday (2100GMT Wednesday).

Democratic Party (DP) candidate Lee Jae-myung cast his ballot in Seoul, while People Power Party (PPP) candidate Kim Moon-soo voted in the city of Incheon earlier in the day.

According to the National Election Commission, on the first day, 19.58% of voters cast their ballots, which is more than 8.69 million out of 44.39 million registered voters.

It marked the highest first-day turnout since nationwide early voting was introduced in 2014.

In a controversial move, members of a group were apparently checking for alleged "Chinese interference" while surveying at some polling stations in Seoul.

They were even using mobile phone cameras to film people entering the polling stations and claiming they are monitoring election fraud, according to Yonhap News.

However, no statement or reaction has come so far from the election commission or local authorities over the group's actions.

Long queues of voters were seen at several polling stations Seoul, although this year's early voting is being held entirely on weekdays, unlike in the past.

Early voting began at 3,568 polling stations nationwide as voters prepare to elect a successor to former President Yoon Suk Yeol for a five-year term.

Yoon was ousted over his abortive attempt to invoke martial law in December.

The two-day early voting period will end at 6 pm local time Friday (0900GMT), according to the National Election Commission.

Currently, six hopefuls are vying for the country's top office, but the actual contest is between two candidates.

The latest poll released Wednesday showed the DP candidate Lee leading with 49.2%, followed by Kim of the ruling PPP with 36.8%.

Lee Jun-seok of the New Reform Party is third with 10.3%.

The presidential election is scheduled for June 3.