SEOUL (Reuters) - South Korean President Lee Myung-bak sacked three ministers on Monday in his first cabinet reshuffle, trying to restore support for his four-month-old government embattled over an unpopular U.S. beef import deal.

Lee replaced his agriculture, health and education ministers. The three have been blamed for policy blunders such as the beef deal reached in April that helped drive public support for his conservative government to below 20 percent.

Lee has seen nearly daily protest rallies starting in early May sparked by a public concerned over mad cow disease that later turned to a broader attack on his pro-business policies.

The cabinet had offered to resign en masse a month ago as protests intensified. There had been speculation in local media that Lee might also sack his foreign and finance ministers.

Lee, who scored a landslide win in a December election, said in an interview with Japan's Kyodo news agency on Sunday that if protests continue, it would be a "detrimental" factor dragging down Asia's fourth-largest economy.

South Korea recently slashed its 2008 economic growth target to a three-year low of 4.7 percent from 6 percent previously as the commodities-led inflation bites into consumer spending and corporate investment.

On Saturday, about 50,000 people protested peacefully in Seoul against the beef deal and Lee's policies, far smaller than the half a million organizers had hoped for.

On Sunday, protesters could not rally enough people for a street march through central Seoul for the first time in over a month, in what analysts said may be a sign that mass street rallies are dying down.

(Reporting by Jack Kim, editing by Jon Herskovitz)

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