The defendant, a Tunisian man with a bushy beard, sits inside a bulletproof glass box in the courtroom. Since his arrest more than a year ago, German authorities have declared the suspect, Ihsan Garnaoui, to be a terrorist and a threat to national security, a man who plotted attacks against U.S. and Jewish targets here.

But since his trial began earlier this month, prosecutors have struggled to make their accusations stick. Witnesses for the state have displayed shaky memories. Security officials have refused to allow two confidential informants to take the stand. And a key police report is missing.

The evidence has been so thin that prosecutors have been unable to provide basic details of the attacks Garnaoui was allegedly planning, such as where they would take place or who else was involved. One of the defendant's attorneys, Michael Rosenthal, wears a happy grin in court and confidently predicts an acquittal. "There's nothing there," he said.

The trial already bears the hallmarks of many other failed terrorism prosecutions across Europe that followed the Sept. 11, 2001, attacks in the United States. European governments have rounded up hundreds of suspects, claiming to disrupt numerous spectacular attacks in the making, only to see the cases collapse months or years later in the courts.

Officials say that difficulties in investigating secretive terror cells, limited cooperation from intelligence agencies and judicial safeguards of defendants' rights have all contributed to this outcome. Muslim spokesmen and civil liberties groups say that police and prosecutors under intense pressure for results often simply go after the wrong people.

European governments have deeply criticized the Bush administration's decision to keep hundreds of terrorism suspects out of the civilian judicial system and put them instead in the custody of U.S. military or intelligence agencies in places such as Guantanamo Bay, Cuba. Such tactics are gross human rights violations, many officials here say. But their own approach has produced few convictions.

In Italy, nine Moroccans who had been held for more than two years on charges of conspiring to poison the water supply of the U.S. Embassy in Rome were acquitted last month after prosecutors admitted they lacked evidence against most of the defendants. Two days later, in a separate trial, three Egyptians were cleared of charges that they intended to bomb Rome's Fiumicino Airport and an American military cemetery.

Those verdicts followed a bungled case last year in which 28 Pakistani men in Naples were exonerated of police claims that they were involved in a convoluted plot with al Qaeda and the Mafia to assassinate a British admiral.

"The reports are completely exaggerated and they create the impression that there is a threat when there isn't one," said Homza Roberto Piccardo, national secretary of the Union of Islamic Communities in Italy. "Muslims come to Italy thinking there is legitimate law enforcement, but those expectations are immediately betrayed."

In Spain, a magistrate in charge of investigating terrorism has indicted dozens of people linked to the Sept. 11 hijackings, but has yet to convict any of them on those charges. In the Netherlands, prosecutors have lost two major terrorism cases, including an alleged conspiracy to bomb the U.S. Embassy in Paris, after judges ruled that evidence obtained by spy agencies was inadmissible in court.

France and Britain have some of the toughest anti-terrorism laws in Europe, enabling them to detain suspects for lengthy periods without trial. But they, too, have had difficulty achieving convictions.

In Britain, 544 people were arrested under anti-terrorist legislation between Sept. 11, 2001, and this January, according to figures provided to Parliament. Total convicted so far: six.

Barry Hugill, a spokesman for Liberty, a British civil liberties group, said authorities could not blame the outcomes on legal technicalities or sympathetic judges. "Given the current climate, the current fear of terrorist attack, getting convictions would not be difficult if there's even a shred of evidence," he said.

In some cases, police or security agencies are quick to make arrests based on rumor or misinterpreted intelligence, as in the case of 10 people arrested last month on suspicion of planning to blow up the stadium of the Manchester United soccer team.

The suspects were released a week later, after authorities determined they were simply sports fans, not Islamic fanatics. "Anyone who just accepts without question what the security services say, we think is very, very naive," Hugill added.

At the same time, European authorities have been less aggressive than American investigators in the pursuit of some well-known radicals.

U.S. officials unsealed a federal grand jury indictment last week against Abu Hamza Masri, a radical London cleric, accusing him of orchestrating a hostage-taking plot in Yemen, among other crimes. The case involved the 1998 kidnapping of 16 Western tourists, a dozen of whom were British.

British officials have long considered Hamza a public menace because of his outspoken support for al Qaeda and have sought to strip him of his citizenship, possibly so he could be deported. But they have never been able to develop a criminal case against him, or to take him into custody until last week. And that was only in response to a U.S. request for his extradition.

On Friday, British Home Secretary David Blunkett said U.S. officials had simply been able to assemble more evidence against Hamza. "If we had that evidence and it related to our country," Blunkett told BBC radio, "we would have been able to take action through our courts."

In Germany, where the government estimates that more than 30,000 people belong to radical Islamic groups, the biggest targets have similarly remained beyond the reach of the law.

A German court last year did convict a Moroccan man, Mounir Motassadeq, of more than 3,000 counts of accessory to murder for aiding the Hamburg al Qaeda cell that carried out the Sept. 11 hijackings. But that verdict was overturned in March by federal appellate judges, who ruled that he was denied a fair trial and deserved a new one. Another alleged 9/11 accomplice, Abdelghani Mzoudi, was acquitted outright in February.

In the Motassadeq case, the appellate court threw out the verdict in part because U.S. officials would not allow testimony or interrogation transcripts from Ramzi Binalshibh, an al Qaeda leader and accused ringleader of the Sept. 11 plot. The defendant's lawyers had argued that Binalshibh could have verified that their client was unaware of the hijackers' plans.

As a result, some Germans have blamed the United States for the outcome of the case and the fact that Motassadeq remains a free man.

"We have a huge problem with the behavior of the U.S. authorities," said Ulrich von Jeinsen, an attorney representing Americans who lost family members in the Sept. 11 attacks. "It is a question to the American side: What are they willing to give us? It is simple and easy. We will have a reluctance [to pursue other cases in court] unless we have an exchange of cooperation among intelligence services."

Some legal experts, however, said German prosecutors and intelligence agencies should be held at least equally accountable. Christoph Safferling, a criminal law professor at the University of Erlangen-Nuremberg, said the appellate judges wanted to send a signal that the German judiciary should be more skeptical of evidence in future terrorism cases.

"When you read the decision handed down, it is in some passages quite angry," Safferling said, referring to the overturning of the Motassadeq verdict. "It is quite angry that this person was convicted on such weak evidence, and also very angry with the intelligence services' [lack of] cooperation."

"The prosecutor was not well prepared in this case," he added. "They were relying on a lot of assumptions and hypotheses but couldn't prove them. I think they were under political pressure. I think if the prosecutor had really thought about it, maybe he wouldn't have indicted in the first place."

Public sentiment is building to change laws in an attempt to bolster security. An April poll by the Allensbach Institute found that 57 percent of Germans surveyed feared that there would be terrorist attacks in the country in the near future, the highest level recorded by the firm since shortly after the Sept. 11 hijackings.

Last week, after years of debate, German political leaders reached a compromise on a new immigration policy that among other things will make it easier for the government to deport terrorism suspects and keep them under closer surveillance.

"It needs to be possible to remove these people from Germany," said Reinhard Grindel, a member of the German Parliament from the opposition Christian Democrats. "There were holes in the laws here, and [the new immigration law] will now close them. The political consequence is that these people will no longer be able to stay in Germany."

But some scholars said it was unlikely that Germany would take stronger steps to expand police powers or allow indefinite detentions, in view of memories of Nazi rule and the Gestapo.

Special correspondents Shannon Smiley in Berlin and Stacy Meichtry in Rome contributed to this report.

Judicial officers stand in a Berlin courtroom next to a glass box where Ihsan Garnaoui, a Tunisian being tried on terrorism charges, sits during his trial.