CAIRO -- Al Qaeda has vowed to keep fighting the United States and avenge the death of Usama bin Laden, which it acknowledged for the first time Friday in an Internet statement apparently designed to convince followers that it will remain vigorous and intact even after its founder's demise.
Al Qaeda's plots are usually large-scale and involve planning over months or even years. But Western intelligence officials say they are seeing increased chatter about cheap, small-scale attacks -- perhaps by individuals or small extremist groups inspired to take revenge for the killing.
"USA, you will pay!" chanted more than 100 participants in a pro-bin Laden protest outside the U.S. Embassy in London on Friday.
A Western intelligence official said no concrete threat has emerged so far that authorities considered credible. "There have been mentions of shootings, bombings and random violence, though it is not surprising, given bin Laden's death," the official said, speaking on condition of anonymity.
Authorities in the U.S. and Europe chose not to elevate threat levels.
Interpol has asked law enforcement agencies in 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.
Despite the Internet chatter, reaction in the Islamic world to bin Laden's death has been relatively muted compared with the rage that he long inspired, raising questions about his relevance in the Middle East -- a region that has been changed by a wave of pro-democracy uprisings.
The Al Qaeda statement, entitled "You lived as a good man, you died as a martyr," did not name a successor to bin Laden. His deputy, Ayman al-Zawahri, is now the most prominent figure in the group and a likely contender to take his place.
"The blood of the holy warrior sheik, Usama bin Laden, God bless him, is too precious to us and to all Muslims to go in vain," the statement said. "We will remain, God willing, a curse chasing the Americans and their agents, following them outside and inside their countries."
"Soon, God willing, their happiness will turn to sadness," it said, "their blood will be mingled with their tears."
Although the statement's authenticity could not be independently confirmed, it was considered to be authentic. It was posted on militant websites Friday by the al-Fajr Center, Al Qaeda's online media distribution network, and the writing style was typical for Al Qaeda. The statement was issued in the name of the organization's General Command and dated Tuesday, the day after bin Laden's death.
White House press secretary Jay Carney said U.S. officials are aware of the statement and the threat. "What it does obviously is acknowledge the obvious, which is that Usama bin Laden was killed," said Carney. "We're quite aware of the potential for (terrorist) activity and are highly vigilant on that matter for that reason."
Despite the new threats against the United States, the overall theme of the Al Qaeda statement was that of continuity for the organization. Much of the 11-paragraph statement was dedicated to underlining that Al Qaeda would live on, depicting him as another in a line of "martyrs" from the group.
"Sheik Usama didn't build an organization to die when he dies," the statement read. "The university of faith, Quran and jihad from which bin Laden graduated will not close its doors," it added.
"The soldiers of Islam will continue in groups and united, plotting and planning without getting bored, tired, with determination, without giving up until striking a blow," the statement said.
It said bin Laden was killed "along an established path followed by the best of those who came before him and those who will come after him."
The acknowledgment by Al Qaeda should remove doubt among all but the most die-hard conspiracy theorists that bin Laden is in fact dead.
The need to provide proof was behind some arguments that the U.S. should release a photo of the slain terror leader. President Barack Obama has chosen to withhold the photo.
Earlier Friday, hundreds of members of radical Islamic parties protested in several Pakistan cities against the U.S. raid. Many chanted "Usama is alive" and criticized the U.S. for violating the country's sovereignty.
In the statement, Al Qaeda also called on Pakistanis to revolt against the country's leaders to "cleanse the shame." And it said that an audio message bin Laden recorded a week before his death would be issued soon.
The writers of the Al Qaeda statement appeared unaware of the U.S. announcement that bin Laden's body had been buried at sea. The statement warned against mishandling or mistreating bin Laden's body and demanded that it be handed over to his family, saying "any harm (to the body) will open more doors of evil, and there will be no one to blame but yourselves."
There had been hope that bin Laden's death would cause the Afghan Taliban to rethink its ties with Al Qaeda -- a union the U.S. insists must end if the insurgents want to talk peace. The foundation of their relationship was believed to be rooted in bin Laden's long friendship with the Taliban's reclusive leader, Mullah Mohammed Omar.
But on Friday, the Afghan Taliban issued a statement saying bin Laden's death will only boost morale among insurgents fighting the U.S. and NATO.
The Taliban praised bin Laden for his sacrifice in the Afghan war against the Soviets in the 1980s and said anyone who believes his death will undermine the current conflict is displaying a "lack of insight."
Al-Zawahri, an Egyptian who is the most likely successor to bin Laden, is a less charismatic, unifying figure. He is believed to lack bin Laden's ability to bring together the many nationalities and ethnic groups that make up Al Qaeda. His appointment could further fracture an organization that is thought to be increasingly decentralized.
Al-Zawahri has long been considered the operational head of Al Qaeda while bin Laden was assumed to be an inspirational figure who was uninvolved in operations.
But documents Navy SEALs seized in Monday's raid on the hideout in the Pakistani town of Abbottabad suggest that bin Laden may have been more involved in operations than had been thought.
The documents reveal plans for derailing an American train on the 10th anniversary of the Sept. 11, 2011 attacks. Counterterrorism officials said they believe the plot was in the initial planning stages at the time.
Al Qaeda, which carried out the attacks on the World Trade Center and the Pentagon, has never abandoned its hope of again attacking the U.S. homeland.
U.S. Sen. Susan Collins, a Republican from Maine, urged that the country's threat level be stepped up while the material seized from bin Laden's compound is reviewed.
In Europe, security officials said there is no specific plot to justify raising the threat level.
British cleric Anjem Choudary, who helped organize Friday's demonstration outside the U.S. Embassy in London, said revenge attacks in Britain and abroad were likely. 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 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.