Support for Mr Lee plummeted over his decision to resume US beef imports
South Korean President Lee Myung-bak has apologised to his country for a beef deal with the US that sparked weeks of street protests.
In a televised address, Mr Lee said he had backed the deal to help secure passage of a bilateral free trade deal, thus boosting South Korea's economy.
But he said he was "deeply sorry" for ignoring concern over health risks.
The protesters say Mr Lee's decision to resume US beef imports fails to protect them from BSE, or mad cow disease.
South Korea used to be a major market for US beef but suspended most imports in 2003 after a case of BSE was detected there.
Mr Lee agreed in April to resume most imports, but protesters say the deal does not provide adequate safeguards against diseased US beef.
"I and the government are deeply sorry" for failing to heed public concerns, said Mr Lee in his address, reported AP news agency.
The president said he "was in a hurry after being elected president, as I thought I could not succeed unless I achieve changes and reform within one year after inauguration.
"As a president, I did not want to miss this golden opportunity," he said of the trade agreement with Washington.
Mr Lee said he had realised his mistake while watching lines of candles filling city streets after dark during one of the protests.
He said he would ensure no US beef from cattle more than 30 months old would reach South Korean plates - a voluntary additional guarantee which Seoul's top trade official is currently attempting to secure in Washington.
Older beef is thought to be at greater risk of BSE.
But President Lee added a note of warning to protesters, saying there was "no possibility" of the trade agreement being ratified this year if South Korea continued to reject American beef and demanded complete renegotiation of the deal.
American deputies have tied ratification of the deal to South Korea re-opening its doors to US beef.
Mr Lee was elected last December by a landslide, but now scores an approval rating of under 20%.
His unpopularity has reduced the chances that he will be able to implement other promised reforms including the privatisation of state firms and corporate tax cuts, say analysts.