People have marked the 20th anniversary of the Columbine High School shooting in suburban Denver by attending a remembrance ceremony.

The event ends three days of sombre gatherings honouring the victims and lending support to their families, survivors of the attack on April 20 1999 and the school’s students and staff.

Starting on Saturday morning, a steady stream of visitors stopped at a memorial that sits on a hill overlooking the school.

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People pay their respects at the Columbine Memorial (AP)

The site includes an oval outer wall of stone with plaques featuring quotes from officials and Columbine students and teachers, and an inner ring with plaques for the teacher and 12 students who were killed.

People walked silently through, occasionally stopping to hug a friend or wipe away tears.

Other visitors left cards, bouquets and seed packets for columbines, the Colorado state flower, around the plaques. Sheriff’s deputies patrolled the area on foot and by bike as little league baseball games went on at nearby fields.

Elsewhere, Columbine students, staff and others took part in community service projects, including volunteering at homeless shelters and doing spring cleaning at the homes of senior citizens and elsewhere.

People later gathered for a remembrance ceremony near the school.

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Floral tributes were left for the 13 people who were killed (AP)

Speakers stressed the strength and change that came out of the tragedy. To symbolise that, artist Makoto Fujimura presented a 17th century Japanese tea bowl which was broken but then mended with gold, making it better and more beautiful.

Pastor James Hoxworth urged anyone who was still struggling because of the shooting to reach out for help.

The days surrounding the anniversary remain emotionally fraught for survivors of the attack, including hundreds who escaped the building without physical wounds.

Some describe their response to the month as an “April fog”, dominated by their memories of the sunny Tuesday two decades ago that shocked the world.

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Crosses bearing the names of the victims of the massacre (AP)

This week brought a new burden as federal authorities led a manhunt for a Florida teenager “infatuated” with the shooting.

On Tuesday, authorities published her name and photo after learning she was obsessed with Columbine and had travelled to Colorado and bought a gun. They said she had not made specific threats, but dozens of schools, including Columbine, locked their doors.

More than 400,000 children stayed home on Wednesday when schools shut down across the metro area. The 18-year-old was later found dead in the foothills west of Denver, about 40 miles from Columbine.

Long-planned events marking the anniversary continued as scheduled, beginning with a Thursday evening church service and a community vigil on Friday night at the memorial.

The Columbine perpetrators, who took their own lives during the attack, have inspired cult-like admirers including some who have committed other shootings or were prevented from doing so.

Officials overseeing security at Columbine and other schools in Jefferson County acknowledged the dark interest this week and warned off those who would treat the school as a destination.

“We are not a place to come visit if you’re not a student, if you don’t have business there,” John McDonald, security chief for the school district, said on Wednesday.

“We’re not a tourist attraction, and we’re not a place for you to come and gain inspiration.”

Security remained heightened at Denver-area schools throughout the week.

People who plan to attend the public remembrance ceremony on Saturday afternoon at a park near Columbine also have been warned of security checkpoints. The high school itself is closed to the public.