ATHENS, Greece — Greece appealed to NATO on Friday for help safeguarding the Olympics, as the deadly train bombings in Madrid sent nations around the world scrambling to protect transport networks and sports events from potential terror attacks.

Europe's travel industry — hoping for a rebound after two weak years — worried that Thursday's attacks would scare off tourists, particularly Americans.

"A lot of people were beginning to feel a little less nervous about traveling. There was a sense that the anxiety has passed," Mike Pina, a spokesman for the Travel Industry Association of America. "This will make people a little more nervous."

Police bomb squads and sniffer dog teams expanded patrols at train stations across Greece. France, Poland and the United States heightened security at railways.

Greece asked NATO to provide aerial and sea surveillance against "a chemical, biological and nuclear incident" — boosting Athens' massive security preparations for the Aug. 13-29 Olympic Games, already costing a record $800 million and involving 50,000 police and troops.

At a major trade show in Berlin, Spanish tourism boosters were shaken as they tried to sell the charms of their country, the No. 2 destination for foreign visitors after France. A large black bow of mourning adorned a panel of glossy photographs promoting the dramatic coastal cliffs of the southern Spanish area of Andalusia.

"It's difficult for us to talk about the attacks. We just say, 'This is not the real Spain,' " said Daniel Navarro of the Huelva Tourism Board, one of nearly 10,000 exhibitors at the International Tourism Exchange.

Ten bombs blew up four trains during morning rush hour in the Spanish capital Thursday, killing about 200 people.

Initially, Spain's government blamed Basque separatists for the attack, but the main group seeking a Basque homeland, ETA, on Friday denied any role. Investigators also were examining a claim of responsibility by a shadowy group in the name of al-Qaida.

In Paris, President Jacques Chirac met with his Cabinet in response to the Madrid attacks after raising France's alert level from yellow to orange. The four-tier French system reaches a red and maximum scarlet level.

French water reservoirs were under heightened surveillance, as were "sensitive areas" such as public buildings, subways and airports. Police measures were tightened at the French-Spanish border, dividing Basque regions between the two countries.

Portuguese authorities preparing Euro 2004 soccer championships said security concerns had shifted overnight from rowdy fans to potential terrorist threats. Officials were mulling whether to install metal detectors at stadium gates.

"These bombings make us feel that the world is more insecure. We must ensure security is handled in the best possible way," said Leonel Carvalho, the Euro 2004 security coordinator.

In Germany, authorities in Bavaria state and the city of Duesseldorf said they ordered police patrolling train stations to be especially vigilant, though Chancellor Gerhard Schroeder said the overall threat level to the country had not increased.

Poland — which joined Spain in supporting the U.S.-led war in Iraq — increased security at airports, train stations and other transport points, while in Hungary, police tightened security at the Spanish embassy in Budapest and train stations.

Britain and several other European countries including Italy, Belgium and Ireland have not increased the terror threat level, noting that air travel and transport networks are already heavily guarded after the Sept. 11, 2001 attacks.

"You could put in place the same sort of screening process as for air travel. But the massive inconvenience would be unacceptable for most people," said Kevin O'Brien, a terrorism expert with RAND Europe, a British-based branch of the RAND Corp. not-for-profit research group.

He said much of the anti-terror effort "has to be based on better intelligence and better surveillance, such as CCTV cameras or smart cameras that scan for suspicious behavior or things that look out of place." He said that might include someone wearing lots of clothing on a very hot day, for example.

In New York City, Mayor Michael Bloomberg said authorities moved to increase security in subways and commuter trains. Also, Amtrak said it was increasing patrols by its police and canine units, and that electronic surveillance of bridges and tunnels was intensified.