Published: May 02, 2011 7:58 PM EDT
Updated: May 02, 2011 5:00 PM EDT

LONDON (AP) - Several European nations tightened security Monday and urged citizens to stay vigilant as Interpol warned of possible retaliatory attacks for the U.S. killing of al-Qaida leader Osama bin Laden.

Extra police across the U.K. were put on alert for any indications of terror plots and to watch for lone gunmen, a British official said, speaking on condition of anonymity.

Embassies across the continent saw extra security. The ANSA news agency reported increased security at Rome's Leonardo da Vinci airport, and police were also more visible at the Duomo cathedral in Milan.

Top Italian security officials held an emergency meeting Monday, with news reports saying they decided to step up security at potential U.S. and Pakistani targets.

"The Interpol has said the risk of terrorism increases with bin Laden's killing; I share this concern," Interior Minister Roberto Maroni told reporters.

Even as they congratulated the U.S. government on the operation, many European leaders said the slaying was of symbolic value. They pointed out that the terror cells working in Europe have long functioned independently and may try to avenge bin Laden's death.

"The fight against terrorism and extremism has, of course, not ended with this," German Foreign Minister Guido Westerwelle said in Berlin. "We must continue to be vigilant and we naturally cannot rule out there being a backlash."

Westerwelle's comments were echoed by British Prime Minister David Cameron, who cautioned that the country needs to be "particularly vigilant in the weeks ahead" in a televised statement.

Defense Secretary Liam Fox said he had ordered all British military bases at home and abroad to maintain a "high level of vigilance" because of possible attacks from al-Qaida or its sympathizers. The government also warned Britons abroad to be cautious in public places and to avoid demonstrations and large crowds.

However, like Germany, Britain's Home Office had no plans to change the country's current overall threat level from international terrorism, which now stands at "severe" - the second-highest threat level, which means that a terrorist attack is highly likely.

"There is nothing at this point that would justify us changing it in the short term," the British official said, speaking on condition of anonymity because of the sensitivity of the issue.

Although there was some speculation that the American operation was behind the decision of Prince William and Kate Middleton to remain in the U.K. after their wedding Friday, a palace spokesman said Monday the couple had made the decision to stay "weeks ago."

In Cyprus, police said they stepped up security at all the island's entry and exit points.

Despite the precautions, some Europeans still were worried about vengeance attacks.

"I am concerned about what reaction his group of followers might have," said Manuel Rabadan, standing outside Madrid's Atocha rail station. "I really am afraid that unfortunately there will be an attack, more or less imminently."

The Madrid rail station was one target in Europe's worst Islamic terror attack, the 2004 Madrid commuter rail bombings that killed 191 people.

In Germany, which had arrested three suspected al-Qaida members Friday on suspicion of making a shrapnel-laden bomb to attack a crowded place, there was no visible beefing up of security.

Interior Minister Hans-Peter Friedrich said the death of bin Laden will weaken terrorism in the medium to long-term but noted that the al-Qaida leader only represents "a symbolic figure" to the terror networks in Europe.

"We must be clear that terrorist cells working in Europe and beyond exist very autonomously, will continue to exist, and we still have a high threat level in Germany and Europe," he said in Munich.

Denmark's intelligence agency also warned about possible backlash attacks, although it said they are likely to be executed by individual and smaller groups outside Europe, especially in Pakistan.

"For militant Islamists, there will be prestige in being the first to avenge him and demonstrate that Islamic militants continue to operate," it said in a terror assessment Monday. "It is (our) assessment that the threat will be especially directed against U.S. interests."

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