Hundreds of protesters have gathered outside the US embassy in London to demand an end to mass shootings, pledging to "stay angry" until gun laws are reformed.
It comes as tens of thousands are set to march on the White House on Saturday in a rally organised by survivors of the Parkland school shooting in Florida.
Student Courtney Chatterton, 21, told the Press Association about a false alarm at her university near Chicago, Illinois, on March 14, the day before she flew to visit a friend in the UK.
She said: "We got all these emergency messages saying there was a shooter on campus. It was utterly terrifying, I started shaking immediately.
"People were barricading themselves in classrooms."
The March For Our Lives movement is urging Congress to ban assault-style, high-capacity rifles which have been used with deadly efficiency in recent mass shootings.
They also want "background check loopholes" closed that allow dangerous people to "slip through the cracks and buy guns online or at gun shows".
Protesters chanted "Not one more" and "books not bullets" before they staged a three-minute silent "die-in" in remembrance of shooting victims.
Organiser Stephen Paduano urged the crowd to "stay angry" at Congress over its "dereliction of duty".
Julia Langfitt, from Maryland, who moved to Surrey with her family two years ago, told the Press Association: "It's time. It's ridiculous how many children are killed every week and still nothing is done about it."
Her daughter Kathrine, 16, added: "My childhood friends go to class knowing they could be killed. No-one should ever be that scared at school.
"They had a lockdown the other week - they happen all the time. They're terrified that this is the new normal."
Around 800 protests are happening worldwide on Saturday, sparked after a former pupil wielding a AR-15 rifle gunned down his ex-classmates at Stoneman Douglas High School.
He massacred 17 students and teachers in the Valentine's Day spree.
The marches follow synchronised national school "walk-outs" across the US earlier this month, when thousands of pupils left class for 17 minutes - one minute for each victim.