Japan issued a travel alert for Europe on Monday, joining the United States and Britain in warning of a possible terrorist attack by al-Qaida or other groups.
Security officials say terrorists may be plotting attacks in Europe with assault weapons on public places, similar to the deadly 2008 shooting spree in Mumbai, India, that killed 166 people, and they urged travelers to be vigilant.
In Washington, U.S. officials said Osama bin Laden and the top al-Qaida leadership were likely behind the plot, adding that the decision to issue the alert was based on an accumulation of information, rather than a specific new revelation.
The U.S. State Department travel alert said public transportation systems and other tourism-related facilities could be targets, noting that past attacks had struck rail, airline and boat services.
'We're not saying don't travel'
The U.S. alert falls short of a more severe one in which the State Department might have warned citizens against traveling to Europe. Instead, the alert urges them to take precautions when they do travel.
"We're not saying don't travel to Europe. We are not saying don't visit ... major tourist attractions or historic sites or monuments," Patrick Kennedy, undersecretary of state, told reporters on a conference call.
"We have a gradation of travel alerts," State Department spokesman P.J. Crowley said Monday. "We have specifically said continue with your travel plans, but be cautious because we are aware of active plots against the United States and allies."
According to an intelligence bulletin obtained by The Associated Press, the FBI and Homeland Security say they have no indication that terrorists are targeting the U.S. or its citizens in particular as part of the new threat against Europe.
On Monday, French authorities arrested a man in his 50s who is suspected of several bomb threats in Paris, including one at a railway hub, a police official said. The man, who was not identified, was detained southwest of the capital on suspicions of links to a phoned-in threat at the Saint-Lazare train station.
French authorities have recorded nine bomb alerts in the capital last month, including at the Eiffel Tower — a threefold increase from a year earlier. No explosives were found.
Amid increased security in Paris, 61 soldiers from an Alpine regiment were deployed over the weekend at two sites in Paris, Notre Dame Cathedral and the Sacre-Coeur Basilica, the joint staff of the Paris defense zone said.
Britain has raised the terrorism threat level in its advice for citizens traveling to Germany and France to "high" from "general." It left the threat level at home unchanged at "severe," meaning an attack is highly likely, and said it agreed with the U.S. assessment for the continent as a whole.
In Rome, speaking on state-run RAI TV, Italian Foreign Minister Franco Frattini said the U.S. alarm about the potential for a terror attack in Europe was "realistic" for Italy because it has troops in Afghanistan. Frattini said there were no specific Italian targets.
Con Coughlin, terrorism expert and executive foreign editor for Britain's Daily Telegraph newspaper, said on NBC's TODAY show Monday that a Mumbai-style attack would be difficult to stop.
"It's impossible to prevent these attacks if you've got these low-intensity terror cells that just want to pick up a gun and start shooting people," he said.
Coughlin also emphasized that travelers should be careful but realistic.
"We can't let the terrorists dictate our lives, and people need to get on with their plans and just be a lot more vigilant," he said.
European authorities tightened security in the wake of the weekend's warnings. However, the German government played down the threat on Monday, saying there were no immediate signs of a plot against Germany.
"There is no reason whatsoever to be alarmist at the moment," Interior Minister Thomas de Maiziere told reporters.
Former U.S. Homeland Security Secretary Michael Chertoff urged Americans in Europe to take commonsense precautions.
"Don't walk around with the American flag on your back," Chertoff told ABC's "Good Morning America." "(Consider) where would you take shelter if something happened."
Vacations plans unchanged
Business travelers and tourists arriving at Paris' Charles de Gaulle airport from the United States on Monday said they were aware of the new warnings from authorities but weren't changing their plans.
"I'm very happy to be here in France. I think we're very safe, and I trust the French government to keep us safe," said James O'Connell, a 59-year-old from Pittsburgh, arriving in Paris for a 7-day vacation.
Karen Bilh, a 39-year-old traveler from Cincinnati, arrived Monday in Paris for a vacation. "We'll pay extra caution and if there's terror threats, we'll listen to police in the area. We're excited about the trip," she said.
Travelers taking the Eurostar trains between London and Paris were similarly determined not to let the warnings disrupt their plans.
Jennifer D'Antoni, who owns a retail clothing store in Britain, was in Paris to celebrate her birthday.
"I had a wonderful time and I'll come back again. In fact, I wish I was here for another day because I didn't get to see everything. We are just going to be a bit more cautious getting on the train," she said.
The last successful large-scale militant attacks in Europe were the July 2005 suicide bombings on London's transport system, which killed 52 people. Bombers killed 191 people on trains in Madrid in March 2004.