The Chicago trial of a fund-raiser for Democratic presidential candidate Barack Obama has opened with the prosecutor alleging massive fraud.
Rezko is accused of using political influence to solicit bribes
The prosecutor alleged that Antoin "Tony" Rezko was behind a $7m (£3.5m) plan to extort money from firms trying to do business with Illinois state.
Mr Obama has not been accused of any wrongdoing and has donated the money raised for him by Mr Rezko to charity.
Mr Rezko denies the charges, saying he only raised funds for politicians.
Mr Obama's links to Mr Rezko have brought the first whiff of scandal to an election campaign that has rapidly gained momentum.
The Illinois senator bought land at a discount from his friend Mr Rezko in 2005, when the businessman was already being investigated.
Mr Obama later called the deal "boneheaded".
Besides raising $150,000 (£75,000) for Mr Obama's election campaign, Mr Rezko was also a major fund-raiser for Illinois Governor Rod Blagojevich.
"When Rod Blagojevich was running for governor in 2002, the defendant was one of his biggest fund-raisers," federal prosecutor Carrie E Hamilton said in her opening statement on Thursday.
"After he was elected, the defendant Rezko was one of his advisors."
He then used his influence to place associates on key state boards, including the powerful Illinois Health Facilities Planning Board, Ms Hamilton said.
In one example, the prosecutor said, Mr Rezko planned with one of the health board members, named Stuart Levine, to get bribes in return for board approval for a hospital expansion.
Mr Rezko and Mr Levine then planned a similar operation with the board of a teachers' pension fund, she said.
"Rezko was the man behind the curtains, pulling the strings," Ms Hamilton said.
Passion for politics
Mr Levine has pleaded guilty in the case in a plea deal with the prosecution.
Mr Rezko's defence attorney, Joseph J Duffy, attacked Mr Levine's credibility as a witness, describing his drug use and calling him "a very, very sophisticated con man".
Mr Duffy said Mr Rezko, a Syrian-born businessman, wanted nothing for himself and raised money for his favourite politicians out of a passion for politics.
Mr Rezko is charged with multiple counts of fraud, of aiding and abetting in the solicitation of bribery, money laundering and attempted extortion, AFP news agency reported.
The trial is expected to last for several months.