THE HAGUE, Netherlands (AP) — An anonymous call from a Canadian phone booth forces a Pakistan-bound airliner to make a nine-hour stop in Stockholm.

A mysterious warning from a Belgian cell phone shuts down a shopping district in Amsterdam for a whole day.

A Yemeni's gifts for relatives arouses suspicion and lands him — and another Yemeni he's never met — in a Dutch jail for two days.

These and many other recent episodes proved to be false alarms, yet paralyzed life for hundreds, perhaps thousands, of people. More than nine years after 9/11, intelligence agencies, police and airlines are still trying to balance protecting society from possible terrorism without derailing routines.

"It's disturbing the way these kinds of situations can disrupt day-to-day lives," said Bibi van Ginkel, a fellow at the International Center on Counter-Terrorism in The Hague.

Authorities are unlikely to let down their guard any time soon, she said, because "they know that these kinds of little tips could easily be a real threat." But she added that protocols are constantly being reviewed to weed out cranks and hoaxers.

The latest false alarm played out Saturday in Sweden when Stockholm police evacuated a Pakistan International Airlines jet and a SWAT team arrested a Canadian man after an anonymous caller in Canada tipped-off authorities that the suspect was carrying explosives. None were found, but passengers waited nine hours before continuing their flight,

The suspect was released Saturday night. Royal Canadian Mounted Police spokesman Sgt. Marc LaPorte said police are investigating whether the call was a "terrorism hoax."

Swedish police defended their actions Monday. Spokesman, Kjell Lindgren said the Swedes acted according to guidelines set out for airline security when they let the plane land in Stockholm.

"With the information that we had, and when the captain doesn't feel he can guarantee the security onboard, I think we did exactly what should be done and it was handled according to the plans that we have," he said.

Lindgren said police followed protocols, but also look to update them. "It is not the case that one doesn't learn from previous incidents and try to find a suitable outlook and suitable measures for every single occasion," he said.

Meanwhile, Paris police on Monday briefly evacuated a busy train station in a bustling area near Paris' historic Opera house following an anonymous bomb alert. Bomb squads and lab technicians turned up no explosives in a search at the Saint-Lazare station.

The difficultly for authorities lies in the fact that threats range from the ridiculous to the terrifyingly real.

In February, Irish Republican Army dissidents and a cab driver called in warnings to Irish police that a hijacked taxi carrying a bomb had been parked outside a Londonderry police station. Nobody was injured in the blast even though the explosives detonated some 20 minutes earlier than the bombers said it would. Police were evacuating people from nearby night clubs as the bomb went off.

On the other extreme, a German traveler running late two years ago for his flight in Verona, phoned in a bomb threat to delay its departure. Authorities closed the airport and rerouted two inbound flights to nearby Brescia before tracing the call to the man's cell phone.

Contrary to widespread belief, security is not being beefed up across the board, and in some cases measures are being scaled back.

The European Union says it plans to phase out restrictions on carrying liquids onto planes by 2013. Airports in Italy are mulling whether to continue using full body scanners to check passengers on certain flights. Last week, the Italian civil aviation authority's president, Vito Riggio, said results from scanner trials at some airports were unsatisfactory because the devices were "slow and inefficient."

Many threats are believed to be screened out. But police seem to have an unstated rule: When in doubt, act.

Earlier this month a bomb warning at the Eiffel Tower prompted the evacuation of some 2,000 visitors from France's most-visited landmark. No explosives were found.

Last year — on the fifth anniversary of the Madrid train bombings — Amsterdam police locked down a busy shopping and entertainment district next to a football stadium and arrested six men and a woman suspected of being on a bombing mission. The action was based on a warning from an unregistered cell phone in Belgium. All suspects were released without charge after a day of interrogation and searches of their homes turned up no evidence of terror links.

Twice in recent weeks, police at Amsterdam's bustling Schiphol Airport have boarded planes to arrest men suspected of terror links.

In the first case, U.S. Homeland Security asked Dutch police to detain a Yemeni man after spotting a cell phone taped to a bottle of medicine and a knife and box cutters in his luggage before he boarded a United Airlines flight. An initial test showed possible traces of explosives, though a later more accurate test showed none. He was detained and questioned for two days — along with another Yemeni who had sat close to him on the plane.

Earlier this month a British man of Somali ancestry also was arrested while en route to Uganda after a tip from British authorities who had received a phone warning. He, too, was released without charge.

Van Ginkel said 100 percent security is theoretically possible, but it would be a bitter pill.

"It is possible, but it is not going to be a state we like," she said. "North Korea might qualify as having close to 100 percent security, but luckily we don't live there."


Associated Press Writer Malin Rising in Stockholm, Sweden, Jamey Keaten in Paris and Francesca Prosperi in Rome contributed to this report.