Supplies of US beef have been in cold storage since last October
South Korea has formally lifted an import ban on US beef, allowing meat from young cattle to be shipped into the country.
South Korea suspended US beef imports in 2003 after a case of mad cow disease, or BSE, was identified there.
The decision in April to end the suspension sparked street protests and left the new government in crisis.
Ahead of the move, at least 3,000 protesters tried to break through a police barricade in the capital, Seoul.
Police used water cannon and fire extinguishers to drive the protesters back, and scuffles were reported. More than 130 people were arrested.
But the crowds were far smaller than the tens of thousands of people who had turned out for some of the protests.
The lifting of the ban means that about 5,300 tons of US beef, held in cold storage since October, will now be inspected and put on sale.
The checking of the meat could begin later on Thursday, said Yoon Young-koo, a spokesman for the agriculture ministry.
Protests against the lifting of the ban have been continuing
In response, activists launched rallies at storage units across the country, prompting police to deploy in some places.
US beef has been banned since 2003, with the exception of a short period last year when limited imports were permitted.
A agreement on ending the ban was reached in April, and it was due to be lifted in May, but this was twice postponed, as protests continued despite reassurances that the meat was safe.
Last week, after further lengthy negotiations, Washington agreed not to export beef from cattle older than 30 months - which is thought to be at higher risk of BSE.
The beef row has posed a significant challenge to President Lee Myung-bak, who took office in February.
He says he backed the deal to help secure passage of a free trade agreement with the US, thus boosting South Korea's economy.
But last week, he made a televised apology to the nation for failing to appreciate the depth of public concern.
He then replaced seven top aides - a decision he said was aimed at giving his administration a fresh start.