A federal prosecutor charged Monday that Sami al-Arian, a fired Florida professor who was a well-known and outspoken advocate for Palestinian issues, lived a double life for more than a decade as the American leader of Palestinian Islamic Jihad, "one of the most deadly terror organizations on earth."

But at the start of Mr. Arian's trial here on terrorism charges, his defense lawyer countered that the case should be seen as essentially a debate over the First Amendment and Mr. Arian's vociferous anti-Israel positions.

"The outstanding feature in this case is freedom of speech," the defense lawyer, William Moffitt, told jurors. "Dr. Al-Arian's right to speak, your right to hear him, and the ability of the powerful to silence him."

The conflicting portraits of Mr. Arian came as the two sides gave their opening statements in United States District Court here in a trial that is expected to last at least six months. Mr. Arian and three defendants who are being tried with him face terrorism and racketeering charges. Prosecutors say they raised money and helped to organize operations for Palestinian Islamic Jihad, a group based in Syria that is blamed for the deaths of more than 100 people in Israel. The case is one of the Bush administration's most significant terrorism prosecutions since the Sept. 11 attacks.

Mr. Arian, whose pro-Palestinian activities have been investigated by American intelligence officials since 1991, became a controversial figure in Florida in the mid-1990's after making sometimes incendiary statements on the Palestinian issue, including using slogans like "death to Israel." Weeks after the attacks of Sept. 11, 2001, the University of South Florida suspended him because of remarks he made on Fox News, and after a lengthy debate, fired him in 2003 after his indictment.

An anonymous jury is hearing the case, and the marshals service erected yellow barricades around the perimeter of the courthouse for the start of the trial, prompting concerns from defense lawyers, who said the heightened security measures could prejudice the jury against their clients.

About a dozen supporters of Mr. Arian's held a rally in his defense over the lunch break, as his wife, Nahla al-Arian, hailed him as "a good man, a courageous man." A few yards away, an Israeli woman, Yaffa Elharar, who flew to Florida for the start of the trial, stood quietly holding an enlarged photograph of her 18-year-old daughter, who was killed in a 1994 attack that Palestinian Islamic Jihad was blamed for.

In a three-and-a-half-hour opening statement outlining the government's case, Walter E. Furr III, an assistant United States attorney in Tampa, provided new details about what he described as "economic jihad" by Mr. Arian and "an elitist group of intellectuals" with whom he was associated in Tampa. Mr. Furr said the government would show that in a three-year period in the early 1990's, the Tampa group received some $1.8 million from overseas to finance its support for terrorist operations.

Mr. Arian, who was born in Kuwait, became a computer engineering professor at South Florida in 1986 and started a research group and other organizations dedicated to the Palestinian cause. But Mr. Furr maintained that the groups were essentially fronts for a more militant agenda devoted to Palestinian Islamic Jihad, and that Mr. Arian served as the group's American leader and, for a time in the 1990's, was "maybe the most powerful man in the world for the organization."

Mr. Furr acknowledged that the government's long-running investigation produced no evidence linking Mr. Arian or his associates directly to the execution of any terrorist attack in Israel or the occupied territories.

"Other people did that," Mr. Furr said. "They ran the organization to make that happen."

The prosecutor said the Tampa group's role in Palestinian Islamic Jihad was three-fold: to help organize the group's structure and operations, to publicize attacks to try to extort concessions from the Israelis, and to raise money "so they could kill more people."

The opening of the government's case also shone a light on usually secret efforts of the Federal Bureau of Investigation.

Using secret warrants, the government in 1993 began monitoring Mr. Arian's communications, taping some 470,000 phone calls. Much of that information was not shared with criminal prosecutors until 2002, however, and Justice Department officials now credit the law known as the USA Patriot Act for breaking down the wall that had hindered the sharing of such information.

Mr. Moffitt, the defense lawyer for Mr. Arian, challenged central parts of the government's claims in often sarcastic terms on Monday, questioning how a man now seen as so dangerous was allowed to remain free for more than a decade under government monitoring.

As an influential Muslim leader, Mr. Arian was active in politics, and he met with President Bill Clinton and George W. Bush, whom he supported in the 2000 election.

Saying the defendant had access to the White House and senior officials throughout the government, Mr. Moffitt questioned whether "anyone ever really thought that Dr. Al-Arian represented a violent threat to the United States."

Mr. Moffitt suggested that Mr. Arian's arrest was driven more by pro-Israeli politics and post-Sept. 11 pressures than by the evidence.