Fox News reports and provides video from Boston:

The Boston Globe reports on Twitter:

BREAKING NEWS: Multiple people injured near the Boston Marathon finish line after explosion

— The Boston Globe (@BostonGlobe)
April 15, 2013

A local reporter notes:

I saw people's legs blown off. Horrific. Two explosions. Runners were coming in and saw unspeakable horror.

— Jackie Bruno NBC10 Boston (@JBrunoNBCBoston)
April 15, 2013

We were so close it was scary. Felt the force of the explosion. People were fleeing and victims were scattered. pic.twitter.com/3WSlXTFkqY

— Jackie Bruno NBC10 Boston (@JBrunoNBCBoston)
April 15, 2013

UPDATE: Apparent video: