UPDATE 2: Here is a video showing the moment the explosion occurred at the end of the Boston Marathon:

UPDATE 1: Fox News report three people are dead:

Three people have reportedly been killed after two blasts rocked the finish line of the Boston Marathon.

Reports are coming in about a massive explosion at the finish line of one of the world’s most famous road running races – The Boston Marathon. Some publications are saying there was one explosion, others two.

According to the Huffington Post:

An explosion has gone off near the finish line at the Boston Marathon.

A number of users on Twitter have tweeted photos showing smoke and people with serious injuries. Other photos show people fleeing from the Copley Square area in downtown Boston. The headquarters of the race is currently on lockdown.

[more here]

Associated Press also reports numerous injuries, and two explosions:

“There are a lot of people down,” said one man, whose bib No. 17528 identified him as Frank Deruyter of North Carolina. He was not injured, but marathon workers were carrying one woman, who did not appear to be a runner, to the medical area as blood gushed from her leg. A Boston police officer was wheeled from the course with a leg injury that was bleeding.

About three hours after the winners crossed the line, there was a loud explosion on the north side of Boylston Street, just before the photo bridge that marks the finish line. Another thunderous explosion could be heard a few seconds later.

Runner Laura McLean of Toronto said she heard two explosions outside the medical tent.

“There are people who are really, really bloody,” McLean said. “They were pulling them into the medical tent.”

[more here]

A political reporter tweeted missing limbs:

Here is a video: