A girl believed to be around 15-years-old was shot dead in east London on Saturday.
Three males, all under 18, were arrested nearby and taken to an east London police station, where they remain in custody.
Police said the teenage victim was gunned down inside a building at Eastway, near Hackney Wick station, shortly before 4pm today.
Hundreds of Americans held a vigil for the victims of gun violence on the cusp of one-year anniversary of the shooting at Sandy Hook Elementary School, which left 20 children and 6 teachers dead.
The multi-faith service in National Cathedral in Washington DC brought together religious leaders and families of the victims of gun violence.
Many guests took to the stage, talking about their experience of losing a loved one in a gun attack.
Anthony Hardmon spoke about the pain after his 19 year-old sister Ashley was killed after being shot in the back of her head few blocks away from her home in Chicago.
Tom Sullivan showed the audience a picture of his son Alex, who died as he celebrated his 27th birthday with the midnight screening of The Dark Knight Rises in Aurora, Colorado.
America's vice president, Joe Biden, has taken his campaign for tougher gun laws on the road. But millions of Americans will take some convincing.
ITV News' Washington Correspondent Robert Moore has been hearing from those who believe in gun ownership and the right to use weapons to protect their children.
Pro-gun activists held "high noon" rallies across the United States to defend the right to own firearms that they say is being threatened by President Barack Obama's gun-control proposals.
The US debate over gun control erupted in mid-December after a man armed with an assault rifle killed 20 first-graders and six adults at an elementary school in Newtown, Connecticut - the deadliest of a string of US shooting sprees last year.
"We are law-abiding citizens, business owners, military, and we are not going to be responsible for other people's criminal actions," former Marine Damon Locke said to applause at a Florida rally he had helped organise.
Teachers in the US state of Utah have been offered free training in handling guns in the wake of the deadly Newtown school shooting.
Gun-rights activists provided 200 teachers with six hours of training in the use of concealed weapons at an event on Thursday.
Kelly Hansen, a special education teacher from Salt Lake City in Utah, told Reuters, "I feel like I would take a bullet for any student in the school district".
"If we should ever face a shooter like the one in Connecticut, I'm fully prepared to respond with my firearm", she added.
The move comes after the National Rifle Association (NRA) proposed placing an armed officer at every school in the US following the deaths of 20 children and six adults at Sandy Hook Elementary School.
Gun owners traded in their weapons for food vouchers in an amnesty-style event in Los Angeles.
Police officers collected weapons from members of the public in an event which had been brought forward following the Connecticut school shooting earlier this month.
France is "awash with illegal firearms" transported into the country across porous European borders, says the president of Union Francaise des Amateurs d'Armes, a French gun society.
Jean-Jacques Buigne said he did not think the killer responsible for yesterday's attacks had a gun licence.
The killer used a heavy calibre .45 automatic pistol. It would be very complicated to obtain this weapon legally due to strict gun laws, but Buigne says open European borders mean a lot of smuggling takes place. He said:
"It's impossible to obtain authorisation for an arm like that if you're not 'clean'."
"It's very complicated to get a weapon like that legitimately. But in France lots of criminals use illegal arms and no law could change that."