SEOUL, South Korea — The elder brother and mentor of President Lee Myung-bak was arrested on bribery charges early Wednesday, further weakening the political leverage of Mr. Lee, a lame-duck leader already grappling with setbacks in both domestic politics and foreign policy.

With the arrest of his brother, Mr. Lee is the latest in an uninterrupted series of South Korean presidents in recent decades to be disgraced by their own or relatives’ corruption scandals.

The brother, Lee Sang-deuk, a 76-year-old former six-term lawmaker from the president’s party, was charged with taking 600 million won, or about $525,000, in bribes from two bankers.

Prosecutors said the bankers had asked Mr. Lee to help stop government regulators from shutting their savings banks down for lax oversight and capital shortages. The bankers have been charged with embezzlement and bribery, and their banks’ operations have been suspended.