Just moments after the explosions, videos and pictures of the horror began to appear on social media. The Images that instantly showed the magnitude of the attack made the terror and fear palpable.
Social networks were filled with conversations, with celebrities like LeBron James and Paula Abdul offering sympathy to victims. People on Twitter were also urging television networks - and fellow tweeters - to show caution in what they were reporting to avoid inflaming the situation with false details.
The "#PrayForBoston" hashtag was trending for hours on Twitter.
Across the U.S., security was tightened at landmarks, government buildings, transit hubs and sporting events. Law enforcement agencies also urged the public via Twitter and Facebook to report suspicious activity to the police.
Google set up a person finder website to help family and friends of runners or those at the marathon to find who they were looking for.
The Associated Press contributed to this report.
