LONDON — 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 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.

British Prime Minister David Cameron cautioned in a televised statement that the country needs to be "particularly vigilant in the weeks ahead." The government also warned Britons abroad to be cautious in public places and to avoid demonstrations and large crowds.

The Associated Press