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.
A makeup date for Bruins-Senators was not announced; no other events from top professional leagues around the country were immediately called off.
Still, other effects of the 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, and armed police officers posted in front of each dugout at the Padres-Dodgers baseball game in Los Angeles.
More toughened measures are expected as security is calibrated for upcoming major events that draw big crowds including the Kentucky Derby on May 4, and the Indianapolis 500 on May 26.
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."
A spokesman for the NBA released a statement to ESPN that read: "First and most importantly, our thoughts and prayers are with all those affected by the devastating events in Boston. While we don't think it is appropriate to get into specifics, we regularly practice a wide range of state of the art security measures in all of our arenas.
"We will continue to work with our arena and team partners, along with local law enforcement to be proactive around the perimeter of our venues and maintain a high level of vigilance, as always."
Abroad, British police reviewed security plans for Sunday's London Marathon, the next major international 26.2-mile race. It drew about 37,500 runners last year.
The London Marathon's chief executive, Nick Bitel, said race officials contacted the police to discuss security plans "as soon as we heard the news" about Boston. He expressed shock and sadness about the situation in Boston, saying "it is a very sad day for athletics and for our friends in marathon running."
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 hockey game's postponement 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."
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 arriving 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.
An electronic sign, which usually lists departure times, instead read: "We ask all passengers to be as vigilant as possible and alert authorities if anything suspicious."
Said the Celtics: "Our sincere sympathies go out to all those affected by this senseless tragedy. The entire Celtics organization would like to acknowledge the heroic efforts of the brave civilian, police, fire and medical personnel responding today in the City of Boston."
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.
Major League Baseball called the bombings a "horrible occurrence" and said the league is monitoring the situation.
"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," MLB 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 major league stadiums with games Monday.
The New England Patriots released a statement Monday night, saying their "thoughts and prayers are with the families of the victims and all who were injured by the bombings.
"Due to the tragic events, the Patriots have respectfully postponed their pre-draft press conference that was scheduled at Gillette Stadium on Tuesday, April 16. We plan to reschedule the press conference later this week."
Information from The Associated Press was used in this report.