In a coordinated effort to crack down on terror cells, police across Europe have arrested at least 20 people and shadowed dozens of others, piecing together the first clues of what may be a network of terror stretching across the continent and beyond. 

Many of the suspects are believed to have links to Usama bin Laden, the suspected mastermind of the Sept. 11 terror attacks in the United States, authorities said. Some allegedly trained in camps in Afghanistan. Others are believed to have plotted attacks on American interests in Europe — including the U.S. Embassy in Paris. 

No one has tied any of the suspects in Europe to the Sept. 11 attacks, but some were linked to one another. Spanish officials announced Wednesday that six Algerians with connections to other suspects had been detained. 

Spanish Interior Minister Mariano Rajoy said the arrests marked "the collaboration against terrorism between all countries, particularly in this case between the countries of the European Union." 

Spanish authorities said some of the Algerians had been in contact with one of the first terror suspects rounded up in Europe after the U.S. attacks — Nizar Trabelsi, a Tunisian arrested Sept. 13 in Belgium. 

"We contacted Belgium and said, 'Hey, the guy you arrested was in Spain on such and such a date and met with these guys," Madrid Police Chief Juan Cotino said. "That's how the link emerged." 

Trabelsi was planning a suicide attack against U.S. targets on the continent, Cotino said. 

Other arrests have taken place in Britain and France. But just how much information they have provided about the larger European terror network is still unclear and it will take more time to piece together a fuller picture, terrorism experts say. 

French judicial officials say Djamel Begal, a French-Algerian man arrested in Dubai in July, has been key to rounding-up suspects and thwarting attacks. 

France's top anti-terrorism judge, Jean-Louis Bruguiere, traveled last week to the United Arab Emirates to question Begal, who authorities have now tied to other individuals arrested in the Netherlands and Belgium since the attacks. 

French police and judicial authorities, who spoke on condition of anonymity, said it was Begal who tipped them off to plots against the U.S. Embassy in Paris. The information led to the arrests of seven people in France on Sept 21. 

An eighth member of the group who eluded capture was arrested later in Britain, as were several other suspects. 

In London Thursday, Home Secretary David Blunkett told the British Broadcasting Corp. that as many as 11 of the 19 hijackers may have been in Britain in the months leading up to the attacks. 

"Some of them will have passed through, some will have stayed over," he said. "What we do now know is, having identified these people — because we do actually now have the line back to where they were — we can track not only their movements, but those who associated with them. That is the crucial issue," Blunkett said. 

British police say they have received more than 100 requests from the FBI in the United States to trace suspects, witnesses and other people connected with the case. 

Blunkett also said it was possible that those who helped the hijackers could still be operating in Britain. 

British anti-terrorist police continued to question six Iraqi men and one German man found hiding Wednesday in a truck parked outside a Royal Air Force base used by U.S. fighter jets. 

In Germany, officials are focused on three former Hamburg students who are believed to have been on the hijacked planes that hit the World Trade Center and the one that crashed in Pennsylvania. 

In Sweden, police have placed individuals under surveillance who allegedly went to suspected terrorist training camps in Afghanistan linked to bin Laden. 

In many cases, police were trailing suspects long before the U.S. terror attacks. Spain's Rajoy said they had been watching the group of Algerians for several months but lacked sufficient evidence to move in on them. 

"It's an old question. Do you string them up or string them along, hoping you'll eventually get more information?" said Frank Cilluffo, a counterterrorism specialist with the Center for Strategic and International Studies in Washington. "Now, given the impending dangers, they obviously moved as fast as they could."