Prosecutors claim Chen and his wife embezzled $3 million in public funds and accepted bribes of about $9 million in a land deal, the state-run Chinese news agency Xinhua reported.
Chen denies the charges and had been on a hunger strike since Friday, resuming eating on Monday. The judge asked him about his health before proceedings began and Chen reportedly said he felt uncomfortable and had a rapid heartbeat.
The trial has a circus atmosphere, attracting many reporters and a large police force called in to maintain order, Xinhua said.
Chen was president of Taiwan from May 2000 until May 2008.
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