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 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. The officials spoke on condition of anonymity because they were not authorized to speak about the plot.

Secretary of State Hillary Rodham Clinton told reporters Wednesday that the U.S. was working closely with its European allies, but she declined to provide specifics.

“We are not going to comment on specific intelligence as doing so threatens to undermine intelligence operations,” Clinton said. “As we have repeatedly said, we know that al-Qaida and its network of terrorists wishes to attack both European and U.S. targets.”

The Department of Homeland Security would not say Wednesday whether U.S. security has been enhanced as a result of the terror threats in Europe.

Revelations of the plot came just ahead of today’s anniversary of the publication of the Prophet Muhammad cartoons in a Danish newspaper. They also came as Spanish authorities announced they had arrested an American citizen of Algerian origin on suspicion of financing al-Qaida’s North African affiliate.

Mohamed Omar Debhi, 43, was taken into custody Tuesday, although Spain’s Interior Ministry said the arrest was not connected to the terror threat. He is suspected of laundering money and sending some of it to an associate in Algeria, Toufik Mizi, to be passed on to al-Qaida cells in the Islamic Maghreb, an Algerian terror group.

Europe has been a target of numerous Islamic terror plots — the deadliest being the 2004 Madrid train bombings, when 10 shrapnel-filled bombs exploded, killing 191 people and wounding about 1,800.

A year later, suicide bombers killed 52 rush-hour commuters in London aboard three subway cars and a bus.

In 2006, U.S. and British intelligence officials thwarted one of the largest plots yet — a plan to explode nearly a dozen trans-Atlantic airliners.