LONDON -- Pipe bombs, a targeted car crash, a lone gunman: Western intelligence officials said Friday they are seeing increased Internet and phone chatter about cheap, small-scale terror attacks to avenge the death of 9/11 mastermind Usama bin Laden.
More than 100 protesters, meanwhile, gathered Friday outside the U.S. Embassy in London shouting, "USA, you will pay!" and warning of such revenge attacks.
European security officials say there is no specific plot to justify raising the threat level in Europe. But one of their biggest fears is the possibility of a Mumbai-style attack like the 2008 shooting spree that killed 166 people and paralyzed India's business capital for days.
Many of the Indian police who initially responded to the attack were unarmed. Most of Britain's police force is also unarmed, while other European police officers usually carry lighter arms than their North American counterparts.
Interpol has asked law enforcement agencies in some 188 countries to be on alert for retaliatory attacks. Communities have been warned to report anything suspicious. Embassies and some American businesses have added new security measures.
"Is it likely that people who follow bin Laden's ideology will try to avenge his death," one European intelligence official told The Associated Press on condition of anonymity because of the sensitivity of his work. "There is an increase in chatter, but that doesn't mean the threat has increased for now."
Europe has been the target of numerous terror plots by Islamist militants. The deadliest was the 2004 Madrid train bombings, when 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 trains and a bus. And 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.
"There has been a mixture of chatter regarding low-tech attacks but nothing as yet that we see as a credible or imminent threat," said a second western intelligence official, also on condition of anonymity. "There have been mentions of shootings, bombings and random violence, though it is not surprising, given bin Laden's death."
According to computers, DVDs and documents from the luxury compound in Pakistan where U.S. officials think bin Laden had been living for up to six years, Al Qaeda had hoped to attack a U.S. rail network to mark the 10th anniversary of the Sept. 11 attacks. The confiscated materials reveal the rail attack planning as of February 2010.
British cleric Anjem Choudary, who helped organize Friday's demonstration outside the U.S. Embassy, said revenge attacks in Britain and abroad were likely because of bin Laden's importance to Al Qaeda and its affiliates. Choudary used to head the outlawed al-Muhajiroun group and is now a member of the Muslims Against Crusades group.
"I think Britain is more likely to face a 7/7 today than ever," he said in reference to the London suicide bombings that took place on July 7, 2005. "Usama bin Laden was a high-profile leader. If the Americans talk of justice, they shouldn't have killed him. The next attacks will likely be high profile and could very well happen in Europe or in the U.S."
He said he had no knowledge of any planned attacks.
Other protesters said attacks were imminent.
"It is only a matter of time before another atrocity -- the West is the enemy," a man who identified himself as 28-year-old Abu Muaz from London said.
There has also been increased praise of bin Laden on Internet forums in Britain, home to nearly 1 million people of Pakistani or Afghan descent.
Kemal Helbawy, an Islamist based in London, joined an online session on the pro-Brotherhood 'On Islam' website Monday -- the day after bid Laden was killed -- asking that God "treat him generously, to enlighten his grave, and to make him join the prophets, the martyrs, and the good people."
Helbawy came to the U.K. in the 1970s as a spokesman for the Egyptian Muslim Brotherhood. He later co-founded both the Muslim Council of Britain and the Muslim Association of Britain, the Muslim Brotherhood's main British front-group, according to the London-based Quilliam Foundation, an anti-extremist think-tank.
"There is a big push to encourage ordinary Muslims in the west to take up arms spontaneously since Al Qaeda can't organize training camps like it used to," said James Brandon, head of research at Quilliam.
One vehicle for encouraging extremism has been through Inspire Magazine, a slick western-style Internet magazine linked to American extremist Samir Khan and Yemen-based Anwar al-Awlaki, tipped as bin Laden's successor. In previous issues, the magazine has offered such articles as "Make a bomb in the kitchen of your mom" and tips on how to fire AK-47s assault rifles.
Brandon said it has appealed to Muslims in Britain and North America, Brandon said.
In October, the U.S. State Department advised American citizens living or traveling in Europe to take more precautions following reports that terrorists may be plotting attacks on a European city. Some of those details came from Ahmed Siddiqui, a German citizen of Afghan descent who was captured by U.S. troops in Afghanistan in July.
Germany authorities were on heightened alert Friday, saying they expect Al Qaeda will be eager to prove itself in the wake of bin Laden's death. They stressed they were not aware of concrete targets or plans, but were watching for any suspicious activity -- particularly small-scale attacks.
Last month, a 21-year-old Kosovo Albanian Muslim targeted U.S. airmen at Frankfurt airport.
Denmark's intelligence service, PET, said it also believes bin Laden's death could trigger terror attacks, and mentioned Pakistan and Afghanistan as places at risk of stepped-up attacks.
"There will for militant Islamists be prestige in being the first to avenge him and demonstrate that Islamic militants continue to operate," PET said.
The United States has issued a global travel advisory urging Americans to be cautious even though the overall terror attack threat level has remained the same in many countries.
Small-scale attacks remain a concern in places like Denmark, where the Danish newspaper Jyllandsposten printed controversial cartoons of the Prophet Muhammad in 2005.
"We continue to review on an ongoing basis all material seized during the (bin Laden) operation as well as new intel that may be coming in," she said earlier this week. "And I stand ready to issue an alert should intel or information emerge that warrants it."