The suicide attacker killed at least 22 people at a concert packed with children on Monday, in Britain's deadliest bombing for nearly 12 years. Islamic State claimed responsibility.
It was the latest in a series of bloody attacks by Islamists which, police say, show a clear policy of choosing "soft targets" where crowds gather in a relaxed atmosphere - in contrast with the vigilant and controlled regime at an airport or government building.
Hundreds have died in such atrocities over the last two years: at the Bataclan concert hall in Paris, in the Bastille Day truck attack in Nice, at nightclubs in Florida and Istanbul, at a Berlin Christmas market and at other scenes of leisure.
Salman Abedi, 22, found a weak spot in security by detonating his bomb outside the Manchester Arena in northern England as people were flooding out at the end of the performance by pop singer Ariana Grande.
That showed the importance of focussing security resources as much on when crowds leave events as when they enter, said Jeffrey Miller, senior vice president at MSA Security in New York.
It also points to the need to move out the security perimeter to well beyond the queue of fans waiting to enter, in order to pick up potential signs of danger, he said.
That would also help crowds disperse, offering less of a target for bombers. "Even though you can hurt or kill people, you wouldn’t kill the same numbers," he told Reuters.
More technology, such as closed-circuit television, facial recognition software, chemical sensors, sniffer dogs and license plate surveillance would also be needed, he said.
Britain raised its terrorism threat level to the highest and deployed soldiers at key sites. London police said they would review security arrangements for all public events and send extra armed officers to sports events.
Some live entertainment shows in northern England were postponed out of respect for the Manchester victims.
FEAR
Fans of Canadian singer Justin Bieber pleaded for his upcoming British tour dates to be cancelled for the safety of both fans and singer.
Chelsea, winners of the English Premier League soccer title, called off a planned victory parade in London on Sunday in light of the Manchester attack.
Other countries tightened security. France ordered heightened security at sports and cultural events and called for closer coordination between private and public security personnel at events.
Interior Minister Gerard Collomb said security had been stepped up at the star-studded Cannes Film Festival.
"We are taking special measures everywhere," he told BFMTV.
The additional measures mean new costs that must ultimately be borne by spectators, performers, promoters or governments.
Asked who would pick up the bill, Collomb said big sports clubs and organisers of major events could afford to pay for their own security but many smaller festivals planned over the summer could not afford it.
"We will help them ... especially through the presence of security forces," he said.
Paris police chief Michel Delpuech said soldiers and police would step up patrols around theatres in the French capital when shows were beginning and ending.
Stockholm police spokesman Kjell Lindgren said security at Wednesday evening's Europa League soccer final in the city, between Manchester United and Ajax Amsterdam, had not been changed.
But he said extra police from other parts of Sweden had already been called in to help. British and Dutch police were also involved.
Belgium, which has also suffered Islamist attacks, has around 1,250 soldiers guarding its streets. The patrols have cost the state about 100 million euros ($112 million) since being introduced at the start of 2015.
No bags are allowed at major sports events in Belgium. Soldiers and armed police are at football matches and bigger concerts.
TAX ON TICKETS?
New York’s Madison Square Garden, which hosts concerts and professional hockey and basketball games, said it had increased security and heightened its police presence.
New York Police Department spokesman J. Peter Donald said additional heavy weapons teams would be sent to "high profile" locations in the city.
"New Yorkers will also see ... explosive detection dogs, highly trained counter-terrorism officers, and random bag checks at transit locations, among other things," he said in a statement.
Mary Mycka, executive director of the U.S. Stadium Managers' Association, said some members were increasing armed security. The association groups about 500 stadiums including most of those in the National Football League, Major League Baseball and Major League Soccer.
"In general, people are tightening down on their security procedures, typically increasing their armed security presence around exterior perimeters before, during and especially after games," Mycka said.
New York-based security consultant David Yorio recommended greater use of bomb-sniffing dogs. But like all security measures, that costs - as much as $40,000 just to get one animal certified.
Yorio suggested a tax on tickets. "Fifty cents added to every ticket can go a long way in terms of technology for detection and other expenses," he said.
(Additional reporting by Daniel Trotta, Laila Kearney and Gina Cherelus in New York, Niklas Pollard in Stockholm and Philip Blenkinsop and Alastair Macdonald in Brussels; editing by Andrew Roche)
