WASHINGTON  Former Representative William J. Jefferson was convicted Wednesday afternoon of using his office to try to enrich himself and relatives through a web of bribes and payoffs involving business ventures in Africa.

A federal court jury in Alexandria, Va., deliberated for five days before finding Mr. Jefferson, 62, a New Orleans Democrat who served in Congress for 18 years until being defeated in 2008, guilty of 11 of 16 counts of bribery, racketeering and money laundering. He was acquitted of obstruction of justice and violating the Foreign Corrupt Practices Act, which makes it illegal to bribe foreign government officials.

Mr. Jefferson faces more than 20 years in prison under federal guidelines when he is sentenced on Oct. 30, prosecutors told The Associated Press. Prosecutors wanted Mr. Jefferson, 62, detained immediately after the verdict on grounds that his ties to Africa made him a flight risk, but Judge T. S. Ellis III allowed him to remain free on bond, citing his local ties.

Mr. Jefferson’s chief lawyer, Robert Trout, told The A.P. that he would appeal.

In a six-week trial, prosecutors said that from 2000 to 2005, Mr. Jefferson sought hundreds of thousands of dollars in bribes from a dozen companies involved in oil, communications, sugar and other businesses, often for projects in Africa.