, such as next month's Kentucky Derby and Indianapolis 500.
And, only a few days away, the London Marathon.
That event's chief executive, Nick Bitel, said officials in charge of Sunday's 26.2-mile race contacted the police to discuss security plans "as soon as we heard the news" about Boston. He expressed shock and sadness about what happened in Boston, saying "it is a very sad day for athletics and for our friends in marathon running."
The postponement of Monday night's NHL game between the Bruins and Ottawa Senators, and the cancellation of Tuesday's NBA game between the Celtics and Indiana Pacers — both events to be held in Boston — were the most tangible reactions by sports officials to the explosions at the marathon finish line that killed at least three people and injured more than 140 others.
Still, other effects of Monday's explosions could be seen quickly, such as bomb-sniffing dogs sweeping the arena before the doors opened for an NHL game in Nashville between the Predators and Canucks that night, and armed police officers posted in front of each dugout at the Padres-Dodgers baseball game in Los Angeles.
A police officer with a German shepherd patrolled near an entrance to the baseball stadium where Miami hosted Washington on Monday evening — an unusual sight at Marlins Park. Inside, on the field, two Marlins officials gave a security supervisor a briefing about the ballpark's layout.
"We are taking additional precautions and have intensified our security efforts in and around the ballpark to ensure everyone's safety," Marlins spokesman P.J. Loyello said.
Fans heading into the NBA game between the Washington Wizards and Brooklyn Nets in New York waited in long lines as arena security used wands to check people and thoroughly checked bags. A short video clip about what happened in Boston aired on the arena's video screen and a moment of silence was held.
"I got a memo ... from one of our security guys. ... 'Hey you know we're a major event in a big building in a big city. So keep your eyes and your ears open. If you see something let us know,'" Nets interim coach P.J. Carlesimo said.
"Without a question, you're thinking we're also" a possible target, Carlesimo added.
Indianapolis Motor Speedway spokesman Doug Boles said Monday's attack will be a part of future meetings to review what precautions should be taken at the auto race.
"I guess this will bring a new topic or dialogue to those discussions, to see if there's anything more we need to do to prepare with respect to what's happened in Boston," Boles said. "And we will learn more about that over the next couple of days, as the folks in Boston do, and we will be prepared for that."
At the Kentucky Derby, which pulls in crowds approaching 250,000 each year at Churchill Downs Racetrack, security was beefed up recently following the death of Osama bin Laden.
"We are always in close contact at this time of year with the dozens of federal, state and local law enforcement and public safety partners who work with us every year on safety and security concerns for our major events," Churchill Downs spokesman John Asher wrote in an email. "We will be in close and frequent contact with them and rely heavily on their expertise, as we always do, in the hours and days to come."
In Brazil, organizers of the 2016 Rio Olympics said they consider security a top priority and are working closely with the local government on safety issues.
Back in Boston, Bruins President Cam Neely, a former player, said the decision to postpone the hockey game was made after consulting city, state and league officials. He said authorities are still gathering information and "it is vital they have all resources available for their investigation."
Another local team, the New England Revolution of Major League Soccer, posted on Facebook that the father-in-law of goalie Matt Reis was injured at the marathon.
Fans arriving early for the Bruins' game were met by police who were in the area to secure the arena and a nearby federal building. One officer outside the players' parking lot was telling spectators, "The game is canceled. We need you to exit the city safely and quickly."
Moments later, Bruins captain Zdeno Chara drove out of the lot, and not long after that the Senators' team bus left down a side ramp.
Police were searching all bags and people entering the train station below the Bruins' arena.
The Red Sox game had been over for about an hour when the explosions could be heard at Fenway Park, about a mile from where the bombs went off at the finish line.
"The safety of everyone that comes to our ballparks is always our top priority and we will continue to do everything to ensure a safe environment for our fans," Major League Baseball spokesman Pat Courtney said.
President Barack Obama's news conference Monday evening was shown on the video board during batting practice before the Cincinnati Reds hosted the Philadelphia Phillies. There was a moment of silence for the Boston victims at that ballpark and at other arenas with pro games Monday.
Phillies center fielder Ben Revere stuck a piece of white athletic tape to the back of his glove before Philadelphia's 4-2 loss in Cincinnati and wrote on it in black marker: "PRAY for Boston."
"I think everyone was thinking about it," Revere said about the bombings. "It hurts to see something like that happen."
AP Sports Writers Ronald Blum, Doug Feinberg, Antonio Gonzalez, David Ginsburg, Gary Graves, Beth Harris, Gregory Katz, Joe Kay, Michael Marot, Charles Odum, Ira Podell, Steve Reed, Tim Reynolds, Ralph Russo, Steven Wine and Tom Withers, and freelancer Jim Diamond, contributed to this report.