PARIS Euphoria over the killing of September 11 mastermind Osama bin Laden was tempered in the West on Monday by fears of retaliation, and world leaders and security experts urged renewed vigilance against attacks.
Americans celebrated on the streets and U.S. markets rallied on hopes bin Laden's death could ease the threats hanging over much of the developed world -- but even President Barack Obama said that terrorist attacks would continue to be a concern.
Interpol predicted a heightened risk and called for extra vigilance in case followers sought revenge for the killing of the man who became the global face of terror, even if he no longer had tactical control of al Qaeda actions.
Members of militant Islamist forums vowed to avenge bin Laden's death and CIA Director Leon Panetta said al Qaeda would "almost certainly" attempt some form of retaliation.
France's President Nicolas Sarkozy hailed the killing as a coup in the fight against terrorism, but both he and Foreign Minister Alain Juppe warned it did not spell al Qaeda's demise.
British Prime Minster David Cameron also said the West would have to be "particularly vigilant" in the weeks ahead.
As he announced bin Laden's death, Obama said: "There's no doubt that al Qaeda will continue to pursue attacks against us. We must and we will remain vigilant at home and abroad."
"The scourge of terrorism has undergone a historic defeat, but this is not the end of al Qaeda," Sarkozy said, after U.S. forces swooped on a luxury compound where bin Laden was hiding out and killed him, along with four others.
Some security experts fear the 10th anniversary of the September 11 attacks could further incite al Qaeda supporters.
"Whilst we in the West might have the satisfaction of justice having been dealt to a terrorist, many will still see Osama bin Laden as a martyr. Make no mistake: violent jihadists will react to this," Julian Lindley-French of London's Chatham House think-tank told Reuters Insider television.
Roland Jacquard, head of the International Terrorism Observatory in Paris, said the United States would be targeted.
"The way in which he was killed, by a military commando, shows this will have important consequences for the future. It will be a call for Jihad, he will remain a very real-life martyr for the rest of the organization," Jacquard told RTL radio.
Islamic militants prayed the news of bin Laden's death was false, or else vowed revenge in comments on online forums.
"Oh God, please make this news not true... God curse you Obama," said one message on an Arabic language forum. "Oh Americans... it is still legal for us to cut your necks."
A man identified as a prominent member of the jihadist internet community by monitoring group SITE said revenge would be taken for the death of "the Sheikh of Islam."
"Osama may be killed but his message of Jihad will never die. Brothers and sisters, wait and see, his death will be a blessing in disguise," said a poster on another Islamist forum.
Experts fear the only blow to al Qaeda will be psychological.
In Washington, a crowd gathered outside the White House as Obama announced the conclusion of a decade-long manhunt, singing patriotic songs and chanting slogans.
The killing was hailed by George W. Bush, who was president when al Qaeda hijackers slammed airliners into the Pentagon and New York's World Trade Center.
"The fight against terror goes on, but tonight America has sent an unmistakable message: No matter how long it takes, justice will be done," Bush said. New York Mayor Michael Bloomberg said he hoped the news would bring closure to those who lost loved ones on September 11.
The dollar and stocks strengthened and oil, gold and silver prices all fell as markets received an immediate boost from the news. But investors said the trend would likely be short-lived.
The United States and much of Europe is on constant alert for an attack by al Qaeda or affiliated extremist organization.
France will stay on the same "red alert" level after bin Laden's death that it has been on since the 2005 London bombings but could tighten security in certain areas.
France has been extra vigilant since bin Laden slammed its attitude toward Muslims in October, and Juppe on Monday warned French citizens to be careful if traveling in North Africa.
The United States warned its citizens worldwide of "enhanced potential for anti-American violence," advising them to avoid mass gatherings and travel, and Australia issued a similar warning. Iraq's army and police went on high alert, but Spain said it was not increasing its security alert.
Japan said it would step up patrols around its military bases to guard against revenge attacks, and in countries with big Muslim populations, some foreign schools, embassies and other potential targets put extra security measures in place.
India, whose ties with neighboring Pakistan are strained, voiced concern that bin Laden was found at a luxury compound just 60 km (35 miles) from the Pakistani capital Islamabad, saying this suggested terrorists could find sanctuary there.
"Osama bin Laden's death doesn't mean we can relax now and assume the danger is past," Wolfgang Ischinger, head of the Munich Security Conference, told German radio.
"I expect al Qaeda will try to get revenge against the Americans and the Pakistan government... Even if a 'battle' has been won, the 'war' is far from over."
(Additional reporting by Chris Allbritton in Islamabad; Vicky Buffery and Alexandria Sage in Paris; Erik Kirschbaum in Berlin and staff in London, Washington, Tokyo, Baghdad and Dubai; editing by Andrew Roche)