Flags are flying at half-mast across the US state of Colorado as the region marks the 10th anniversary of the Columbine High School massacre.

Two heavily armed students - Eric Harris and Dylan Klebold - entered the school and opened fire, leaving 13 people dead and 23 others wounded.

Relatives of victims and community leaders gathered for a candlelight vigil last night at Littleton’s Columbine Memorial.

Colorado Governor Bill Ritter, who ordered flags on all public buildings across the state to fly at half-mast, said the massacre ‘will continue to live in our memories'.

‘People to this day remember exactly where they were when they first heard about the tragedy unfolding in Jefferson County,’ Governor Ritter said.

The killings of 12 students and their teacher marked a ‘tremendous loss of innocence in America and their memories must not be forgotten,’ he added.

‘We cannot allow the lessons of this tragedy to fade with the passage of time. The families of those who died that day remain in our thoughts and prayers.’

As it has done every year since the massacre, Columbine High School is closed for the day.

The school's long-serving principal, Frank DeAngelis, who has vowed to remain in the job until every student who was in kindergarten on the day of the attack has graduated, is among those attending today's service.

While Columbine was the catalyst for a debate about gun control in the US, attempts to restrict weapons have largely fizzled out.

The Bush administration failed to renew a ten-year moratorium on the sale of assault rifles, which expired in 2004. President Barack Obama's administration has said it plans to renew the ban.

In several mass shootings since Columbine, most notably the 2007 Virginia Tech massacre that left 32 people dead, guns involved were obtained legally.