Voters began casting ballots in parliamentary by-elections Wednesday amid a broadening bribery scandal involving some key aides to President Park Geun-hye.

Four seats are up for grabs in the by-elections widely seen as a bellwether for next year's general elections.

Polling stations in four constituencies, including one in Seoul, opened at 6 a.m. and are set to close at 8 p.m. to allow more voters to cast ballots before and after work.

As of 3 p.m., voter turnout stood at 26.5 percent, with more than 188,000 people casting their ballots, the National Election Commission said.

The by-elections came amid a probe into the bribery scandal that led to the resignation of Prime Minister Lee Wan-koo.

Lee is accused of taking 30 million won ($27,000) from a businessman at the center of the bribery scandal in 2013, when Lee was running for a parliamentary seat.

Before hanging himself earlier this month, the businessman left behind a memo listing the names of Lee and seven other high-profile politicians he claimed to have bribed. All but one listed on the memo are close associates of the president.

Park accepted Lee's resignation offer earlier this week.

The main opposition New Politics Alliance for Democracy, which fielded candidates for all the four seats, is running on its election campaign to pass judgment on the scandal-stricken Park administration.

Expressing regret over Lee's resignation, Park vowed to root out corruption and push for a fresh political reform.

Another major intrigue in the by-elections is the fate of two heavyweight politicians who defected from the main opposition party to run for the parliamentary races.

The two former NPAD members -- Chung Dong-young and Chun Jung-bae -- are each running for the race at the B constituency of the Kwanak district in southern Seoul and the B constituency of the Seogu district in the traditionally left-leaning southwestern city of Gwangju.

The bids by Chung, a former presidential candidate, and former Justice Minister Chun are expected to split opposition votes.

Both the ruling and main opposition parties are running toward the goal of each winning at least two seats, although ceding all four seats to the opposition bloc will not weaken the ruling party's 157-strong majority held at the National Assembly.

The Saenuri Party is largely seen as holding sway over the B constituency of Ganghwa county, Incheon, with the rest of the three, including the Jungwon district in Gyeonggi Province, preferring the opposition bloc.

Voter turnout during the two-day early voting, held last week, stood at 7.6 percent, according to the National Election Commission. (Yonhap)