Two bombs have exploded in the packed streets near the finish line of the Boston Marathon, killing at least three people and injuring more than 100.

There was no word on the motive or who may have launched the attack, and police said no suspect was in custody.

Words of condolence and sympathy from around the world have been offered by leaders following the deadly attack.

Barack Obama, US President

"Make no mistake: We will get to the bottom of this. We will find out who did this and why they did this". 

Deval Patrick, Governor of the State of Massachusetts, US

"This is a horrific day in Boston. My thoughts and prayers are with those who have been injured. I have been in touch with the President, Mayor Menino and our public safety leaders. Our focus is on making sure that the area around Copley Square is safe and secured. I am asking everyone to stay away from Copley Square and let the first responders do their jobs".

Ban Ki-moon, Secretary General of the United Nations

"I condemn this senseless violence, which is all the more appalling for taking place at an event renowned for bringing people together from around the world in a spirit of sportsmanship and harmony”.

David Cameron, Prime Minister of the United Kingdom

"The scenes from Boston are shocking an horrific - by thoughts are with all those who have been affected".

Stephen Harper, Prime Minister of Canada

"It is truly a sad day when an event as inspiring as the Boston Marathon is clouded by such senseless violence. Our thoughts and prayers are with those injured or affected by this horrible incident. We stand with our American neighbours in this difficult time".

Julia Gillard, Prime Minister of Australia

"The images we've seen have been truly shocking ones and our thoughts today are with those who have lost loved ones".

Yoshihide Suga, Chief Cabinet Secretary of Japan

"We express our heartfelt sympathies to the victims and their families of this painful incident. Regarding safety of Japanese nationals, we are currently checking with our consulate in Boston, but we do not have information that there have been Japanese victims. We coordinating closely with the US government, and US relevant authorities have said an investigation is ongoing".

Source: Agencies