LONDON (AP) -- With two high-profile events in the days ahead - Margaret Thatcher's funeral and the London Marathon - British officials are looking anew at security precautions following the bombings in Boston.
Terror threat levels in Europe and elsewhere, however, have remained unchanged, in contrast to other recent bombings and thwarted attacks that raised alarms and travel warnings.
Such warnings have been issued in the past when threats are considered imminent and with potential international links.
Threat levels also remained unchanged at U.S. defense installations at home and abroad after Monday's deadly bombings at the Boston Marathon, according to a Pentagon spokesman who requested anonymity because he was not authorized to speak to the press about security.
Britain made last-minute efforts to tighten measures for Wednesday's funeral for Thatcher, the former prime minister, at St. Paul's Cathedral, which is to be attended by hundreds of diplomats and dignitaries, including Queen Elizabeth II.
The chief of London's Metropolitan Police, Bernard Hogan-Howe, said the force will carry out more searches and put more officers on the streets of the capital in the coming days as a precaution.
Police with bomb-detecting dogs were seen Tuesday around such London landmarks as Big Ben, the Houses of Parliament and Trafalgar Square, but officials said the searches were routine and unrelated to the Boston attacks.
"The (Boston) attacks mean that we will be assessing our security protocols," said a British security official who spoke on condition of anonymity because he is not authorized to be publicly named. "There is some initial information coming out ... but it is too early to draw any conclusions. There doesn't appear at this point, however, to be a wider threat."
More than 37,000 runners and a half-million spectators, including Prince Harry, will be at Sunday's London Marathon. Marathon officials said the race would go on as planned but security was being evaluated.
"The best way for us to react is to push ahead with the marathon on Sunday, to get people on the streets and to celebrate it as we always do in London," British Sports Minister Hugh Robertson said. "We are absolutely confident here that we can keep the event safe and secure."
Security would be particularly tight for the big events in Britain, which has been at the heart of several terrorist attacks in the past decade, including suicide bombings in 2005 that killed 52 people. Several international terror plots have also been traced back to suspects in Britain.
Workers are inspecting some of the country's 4.3 million CCTV cameras in high-traffic areas around London to ensure views are unobstructed and equipment is functioning. Workers in an underground bunker monitor the footage around the clock.
Boosting security may also include adding manpower, increasing air visibility and securing public transport routes. Police and counterterrorism officials are also aggressively monitoring potential suspects.
Security was more evident at sites across the U.S., with military personnel seen near the Pentagon's subway station in Washington, D.C., and officers deployed to Chicago's Union Station.
At the White House, the Secret Service expanded its security perimeter after the attacks, shutting down Pennsylvania Avenue and cordoning off the area with yellow police tape.
In New York, authorities deployed highly visible patrol units that move in packs with lights and sirens along with more than 1,000 counterterrorism officers. Highly trafficked tourist landmarks were being especially monitored.
"No matter how many days, months or years pass without a major terrorist attack, it only takes one such attack to bring us back to the cruel reality," Interpol chief Ron Noble told The Associated Press early Tuesday, saying police around the globe would be on alert.
U.S. Secretary of Homeland Security Janet Napolitano urged the American public "to be vigilant and to listen to directions from state and local officials."
The Boston bombings underscored the security challenges facing next year's Winter Olympics in Sochi, Russia, the 2014 World Cup in Brazil and the 2016 Summer Games in Rio.
Ever since the attacks by Palestinian gunmen that killed 11 Israeli athletes and coaches at the 1972 Munich Games, security has been a paramount concern for the Olympics.
"We are very, very concerned," senior IOC member Gerhard Heiberg of Norway told the AP. "Security is priority No. 1, no question about it."
Brazilian Foreign Minister Antonio Patriota said "all necessary measures" are being taken to ensure security at the World Cup and the Rio Olympics.
Russian officials gave mixed signals over whether they needed to increase security at key sporting events like the World Athletics Championship and the Sochi Games.
The track and field championship, which takes place in Moscow on Aug. 10-18, is seen as a dress rehearsal for Sochi.
One top sports official said security was being increased but others said Russia's take on Olympic security was already very robust.
Officials will speak with Boston Marathon organizers to find out what more precautions are needed, said Mikhail Butov, secretary general of the Russian Athletics Federation, adding that "when it's clear what actually happened (in Boston), we will draw our own conclusions."
Federation President Valentin Balakhnichev told the Interfax news agency that the Boston bombings revealed "problems" in ensuring security at outdoor events and expressed concern that it may inspire "other organizers of terrorist attacks."
In addition to Sunday's event in London, more than 30 marathons are also being held across the world this weekend in countries across Europe, in Japan, South Africa and around the United States.
In Serbia, officials said they would raise their guard for the race.
"We will do our best so that this year the security level is even higher," said Dejan Nikolic, the organizer of Sunday's Belgrade Marathon.
Police in Linz, Austria's third-largest city, said security was being tightened for the city's marathon Sunday, with police closely checking key points along the race, particularly the finish-line area.
Police Col. Heinz Felbermayr told reporters that his units are "totally prepared" for any eventuality, adding that while the Boston bombings should not be played down in relation to Linz "they also should not be overly dramatized."
Organizers of the Hamburg Marathon said they did not plan any changes to their security measures for Sunday's race, saying they will have 400 officers on hand.
Madrid authorities said they will meet next week to decide if extra security measures are needed for the April 28 marathon.
Race officials for the Illinois Marathon in Champaign and Urbana, Illinois, said they were already fielding calls from worried runners and their families and planned to meet Wednesday to discuss more security measures such as bomb-sniffing dogs.