London ? Police increased their guard around Buckingham Palace and other landmarks Wednesday as security officials monitored what they described as a fledgling terror plot to wage Mumbai-style shooting sprees or other attacks on Britain, France or Germany.

At least some of the recent CIA strikes in Pakistan were aimed at al-Qaida operatives suspected in the plot, U.S. officials said. European officials said the plot was still in its early stages and not considered serious enough to raise the terror threat level.

Still, the Eiffel Tower in Paris was briefly evacuated Tuesday — the second time in two weeks because of an unspecified threat — and French police were on alert.

A heavy police presence was seen Wednesday around Buckingham Palace, Trafalgar Square and Big Ben. Victoria Station was briefly evacuated after an unusual smell was reported.

“This plot was in its embryonic stages,” a British government official told The Associated Press, speaking on condition of anonymity because of the sensitivity of his work. He said the plot had preoccupied the security community more than other recent threats, but did not merit changing the security threat level from severe to critical.

Some details about the plot came from Ahmed Siddiqui, a German citizen of Afghan background who was captured in Afghanistan in July, a U.S. official said.

Intelligence authorities used National Security Agency wiretaps to flesh out details, U.S. officials said, and while a Mumbai-style shooting spree was one possibility, there was no concrete plan.