"There's no reason to put New York through this," said Giuliani, who was mayor when terrorists flew two hijacked planes into the World Trade Center and one struck the Pentagon in 2001. A fourth jet crashed in western Pennsylvania.

"If President Barack Obama had concluded that military tribunals were impermissible," Giuliani said on NBC's Today show, "I would have been in favor of it." He called the decision to use the federal court system "unprecedented" and said that in the Civil War and in both World War I and World War II, the United States relied on military courts.

Former attorney general John Ashcroft, in an interview with NBC, said the administration's decision to turn to the civilian court system "may be a new level of repudiation" of the notion that the United States is undertaking a war on terror.

Obama said in network interviews Wednesday that the move was just and that critics of the prosecutorial decision will better understand when Mohammed is convicted. The president then backed off slightly, noting that he would not be a participant in that trial.

His attorney general, Eric Holder, strenuously defended the decision before skeptical members of the Senate Judiciary Committee, voicing confidence there would be a conviction and saying the decision was right.

Giuliani said he had no doubt that authorities would keep New York safe during the trial. But he said there will be "tens of millions of dollars in additional burden" for security, nevertheless. He said that at a previous trial of a terrorist suspect in New York in the 1990s, he had to close the streets to the public near City Hall and in the Wall Street area and that he "was pilloried" for that.

Giuliani, who unsuccessfully sought the Republican presidential nomination last year, told Today: "The reality is, we are breaking precedent here. ... The reality is, he would get a fair trial there (in a military tribunal). A case like this, the government is put on trial. The more exciting headlines will be the headlines against the government. The headlines will be, '180 Waterboardings. The CIA did this terrible thing and that terrible thing to me.' Some of it will be lies and some of it will be true."

In an interview on CBS's The Early Show, he said, "It's a political decision because I believe this is being done to satisfy left-wing critics who all during the last two or three years have campaigned against these military tribunals."

Enlarge By Spencer Platt, Getty Images Former New York mayor Rudy Giuliani speaks to the media at the Molinari Republican Club dinner Monday in Staten Island. Giuliani opposes the Obama administration's decision to try 9/11 suspects in civilian court.