The vehemently homophobic Westboro Baptist Church has threatened to picket the funerals of those killed in the Boston Marathon bomb attack.

Three people died and at least 140 were injured after two explosions went off near the finishing line of the Boston Marathon on Monday.

One of the dead was an eight-year-old boy, US media said.

In a televised address, President Barack Obama vowed to bring those behind the attack to justice.

“We will find out who did this. We’ll find out why they did this,” he said.

“Any responsible individuals, any responsible groups, will feel the full weight of justice.”

Westboro, based in Kansas, tweeted: “BREAKING: Westboro Baptist Church to picket funerals of those dead by Boston Bombs! GOD SENT THE BOMBS IN FURY OVER FAG MARRIAGE! #PraiseGod.”

Another tweet, referring to the Ellen DeGeneres Show, said:

“Everyone can give a big thanks to the likes of ‪@TheEllenShow in supporting same-sex marriage – THAT is the reason GOD SENT BOSTON BOMBS!”

Massachusetts became the first US state to introduce equal marriage in 2004.

Westboro Baptist Church has a long history of using national US tragedies to promote their anti-gay agenda.

The group came into the spotlight in 1998, for picketing the funeral of Matthew Shepard, a gay man who was killed in a homophobic attack in Wyoming.

In 2005, Westboro began its campaign of picketing the funerals of fallen US troops.

The church blamed homosexuality for the Newtown massacre in Connecticut where 20 children and six adults were killed by a gunman at Sandy Hook elementary school in December.