Security agencies across the Western world started beefing up security within minutes of the official announcement of Osama bin Laden's death, warning that al-Qaeda would seek to avenge the death of its leader.
The U.S. put all American embassies and government facilities around the world on a "heightened state of alert" on Monday and warned Americans abroad of possible reprisal attacks.
In a worldwide travel alert released shortly after U.S. President Barack Obama confirmed bin Laden's death in a late Sunday TV address, the State Department said there was an "enhanced potential for anti-American violence."
The alert said Americans travelling or living abroad should "limit their travel outside of their homes and hotels and avoid mass gatherings and demonstrations."
CIA director Leon Panetta said al-Qaeda would "almost certainly" try to avenge bin Laden's death.
'Though bin Laden is dead, al-Qaeda is not.' — Leon Panetta, CIA director
"Though Bin Laden is dead, al-Qaeda is not," Panetta said in a statement. "The terrorists almost certainly will attempt to avenge him, and we must — and will — remain vigilant and resolute."
Canadian Prime Minister Stephen Harper struck a similar note in his statement.
"Bin Laden's death does not end the threat of international terrorism," he said. "Sadly, others will take his place."
Canada upgrades warnings
Late Sunday night, Canada's Department of Foreign Affairs upgraded travel warnings for Afghanistan and Pakistan, urging Canadians to "avoid all gatherings and demonstrations, and to stay away from areas where they may take place."
Transport Canada refused to say whether security was being upgraded at the country's airports.
"Transport Canada does not divulge details related to security operations," it said in an email to CBC News.
An Air Canada spokesman said there was "at present no impact on our customers" and the airline's website said the company anticipated a "normal operational" day.
The RCMP said it was reviewing its security operations to see what effect — if any — bin Laden's death might have on the threat level in Canada. Among other duties, the Mounties provide security around foreign embassies in Canada.
British officials said security would be upgraded at U.K. missions abroad.
"There may be parts of al-Qaeda that will try to show that they are still in business in the coming weeks, as indeed some of them are," British Foreign Secretary William Hague told the BBC on Monday.
"I have already this morning asked our embassies to review their security to make sure that vigilance is heightened and I think that will have be our posture for some time to come," he said.
However, International Olympic Committee president Jacques Rogge said the development would make no difference to organizers of the 2012 London Olympics because security was already so tight.
Bin Laden followers will 'treat him like a martyr'
Security experts said a worldwide boost in security is justified.
"I think [bin Laden's] followers are going to treat him like a martyr and they're going to try and retaliate," said Prof. Andre Geralymatos of Simon Fraser University and a member of Canada's advisory committee on national security.
"This is why every American base, every Canadian base, every embassy is put on full alert," he told CBC News. "And I'm sure people are going to find a lot more security at other places of transit."
Law enforcement officials tightened security at many airports, transit systems and other potential targets around the world.
The U.S. Department of Homeland Security said it was not planning to issue a national terrorism advisory alert.
"We will only issue alerts when we have specific or credible information to convey to the American public," said Homeland Security Secretary Janet Napolitano.
"However, our security posture, which always includes a number of measures both seen and unseen, will continue to respond appropriately to protect the American people from an evolving threat picture both in the coming days and beyond."