World leaders and security experts have urged renewed vigilance against retaliatory attacks following the killing of Osama bin Laden by US troops in Pakistan.
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.
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.
United Nations Secretary-General Ban Ki-moon said bin Laden's death was a key turning point in the world's struggle against terrorism.
"The death of Osama bin Laden, announced by President (Barack) Obama last night, is a watershed moment in our common global fight against terrorism," Mr Ban told reporters.
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, US president Barack 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," Mr Sarkozy said, after US 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," said Julian Lindley-French of London's Chatham House think-tank.
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 organisation," Mr Jacquard told RTL radio.
Experts fear the only blow to al Qaeda will be psychological.
In Washington, a crowd gathered outside the White House as Mr 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 Centre.
"The fight against terror goes on, but tonight America has sent an unmistakable message: No matter how long it takes, justice will be done," Mr 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 United States and much of Europe is on constant alert for an attack by al Qaeda or affiliated extremist organisation.
France has been on red alert, the third-highest level in a four-step scale, since suicide bomb attacks in London in 2005 and has been especially vigilant since bin Laden criticised the country's attitude towards Muslims last October.
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.
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 neighbouring Pakistan are strained, voiced concern that bin Laden was found at a luxury compound just 60 km 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," he said.