Protesters picketed South Korea's top port and other locations to block any shipments of US beef after the government on Thursday formally lifted an import ban.

South Korea officially authorised the resumption of imports after winning new safeguards from Washington against mad cow disease. Fears of the disease prompted weeks of mass street protests which shook the government.

The Korean Confederation of Trade Unions (KCTU) said some 100 of its members picketed the approach to a pier in the southern port of Busan where 3,300 tons of frozen US beef is awaiting inspection and distribution.

"Our fight has begun today to frustrate any distribution of US beef products in the country," KCTU spokesperson Woo Moon-Sook told AFP.

The spokesperson said the confederation had sent members on similar missions to storage facilities across the country. A total of some 5,300 tons is now in Korea awaiting inspection.

Riot police were deployed in Busan and elsewhere but there were no immediate attempts to ship US beef out of the facilities.

In hopes of pushing ahead with a wider trade deal, Seoul signed a deal in April to resume the imports which were halted in 2003 after a US mad cow case.

South Korea has suffered weeks of occasionally violent protests by tens of thousands of people against the resumption of imports, and in protest at government policies in general.

Rallies have been much smaller recently. But after the government announced its plan to resume imports, some 3,000 people tried to break through a barricade of police buses in downtown Seoul overnight on Wednesday.

Police used water cannon and hand-held fire extinguishers to disperse them and detained more than 130 people.

Some demonstrators smashed bus windows and others tried to break down low walls blocking their path in attempts to reach the presidential palace. The two sides exchanged kicks and punches.

In an attempt to quell the protests, Seoul sent negotiators to Washington this month to secure extra safeguards against any risk of mad cow disease.

President Lee Myung-Bak, whose popularity has plummeted, called for an end to the unrest on the streets.

"Now is the time to concentrate our capabilities on economic revival after putting an end to various disputes over US beef imports," he told a special Cabinet meeting after the imports formally resumed.

"A persistent chaotic situation in state affairs will eventually damage the livelihoods of ordinary people," he said.

The public administration and security ministry posted the new beef regulations in the government gazette Thursday.

"The official notice was published online as well as in the paper gazette at 9:00am (0000 GMT) today," a spokesman said.

"With the publication of the new guidelines, quarantine authorities can resume work on US beef imports from now on."

US exporters have now agreed not to send meat from cattle older than 30 months, seen as more at risk of the disease. The US government will verify the age of the exports.

But protesters said the new safeguards are still inadequate and urged people to continue the rallies.

According to a survey commissioned by the independent Hankyoreh newspaper, more than six out of every 10 South Koreans are still concerned about the safety of US beef.