David Cameron has paid tribute to the spirit of Boston's people as he visited the site of the marathon bombing.
"Everyone in the UK stands with your great city and your great people," he told reporters in the square.
Mr Cameron said it was crucial to challenge the "narrative of violent extremism".
That meant "standing for the values that we believe in", such as freedom, democracy and diversity.
"We know how important it is to stand up and say the terrorists will not win," the premier added.
"We will never give in to terrorists."
The tribute came on the second day of Mr Cameron's three-day trip to the US.
However, he has been unable to escape the spectre of Tory jousting over the EU at home.