The Welsh national field of remembrance at Cardiff Castle features 10,000 wooden crosses remembering the fallen.
Each one carries a message to someone who has died while serving their country since World War One.
Royal British Legion members and First Minister Carwyn Jones were among those at the service.
Mr Jones said he was "proud to be attending this event to honour those who have died in conflict."
"As we mark the 100th anniversary of the First World War, there is an even greater opportunity for us to remember those who took part in the War, and the impact of the conflict in shaping modern Wales," he added.
At 11:00 GMT there was a two minute silence during which Royal British Legion buglers played the Last Post trumpet call.
National Chairman of The Royal British Legion, John Crisford, says the service shows the public's gratitude to those who "made the ultimate sacrifice in defence of our country".
A field of remembrance honouring members of the armed forces from the past 100 years is open at Cardiff Castle.