Boston police say at least two people have been killed and 22 injured in the explosions at the finish line of the Boston marathon.

The explosions occurred about three hours after the winners crossed the line, as stragglers were still finishing in the 26.2-mile (42.1 kilometre) trek.

The Boston Marathon says the explosions were caused by bombs and that organizers are working with authorities to determine what happened.

Organizers made the announcement on the groups’ Facebook page.

More than two-thousand Canadians were taking part in the race.

Prime Minister Harper has tweeted that he was shocked to hear the news from Boston and that his thoughts are with those who are affected.