UPDATE 2-S.Korea's troubled president seeks rival as PM

(Adds president's comments)

By Lee Suwan

SEOUL, June 11 (Reuters) - South Korea's unpopular president has approached his main rival in his ruling conservative party to be prime minister to stem anger over policies that have triggered mass street protests.

The rallies calling for President Lee Myung-bak's ouster after barely three months in office were sparked by a public outcry over a deal to open South Korea's market wider to U.S. beef, and have cast a cloud over his plans for pro-business reforms.

Lee has unofficially called on Park Geun-hye, his rival for control of the conservative Grand National Party (GNP), to be his prime minister, an aide close to Park told Reuters on Wednesday.

"There is a chance she could accept if a formal request is made because the situation is so difficult," said the aide, who asked not to be named.

Calling for her help now can be seen as a humbling admission that he needs her strong network of supporters and skill at managing political affairs to fix his government, analysts said.

Lee said at a meeting of small- and medium-sized business owners: "I am determined to make a fresh start."

Park, a former GNP chair and daughter of an assassinated dictator, is expected to bring a steadying hand to Lee's government, roundly criticised for bungling the U.S. beef import deal and being out of touch with public sentiment, analysts said.