Prime Minister Malcolm Turnbull attended the service, laying a wreath with wife Lucy.
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Mr Turnbull spoke of the horror that unfolded at the memorial pool site, which is near the Broad Arrow Cafe where the shooting began.
"Despite the years, despite the healing, the sense of loss weighs heavy. We will never be the same."
- Prime Minister Malcolm Turnbull
"Some came to work, some came to relax and learn. It was to be another calm day amid the sandstone ruins," he said.
"And then the horror.
"Despite the years, despite the healing, the sense of loss weighs heavy. We will never be the same."
EARLIER: Choirs are practicing as people start filtering into the Port Arthur Historic Site for a 20-year commemoration service of the 1996 massacre.
The service will be broadcast live on ABC24 and on Sky News.
Premier Will Hodgman and Acting Police Commissioner Scott Tilyard will speak at a media conference following the one hour service, set to start at 12.30pm.
Mr Hodgman, Prime Minister Malcolm Turnbull and Governor Kate Warner will address the service.