Charity Pompey Pals will hold its fourth annual commemoration in Guildhall Square, Portsmouth, on Saturday from 8am to 5pm.
The event will focus on the 14th and 15th battalions of the Hampshire Regiment – formed by Portsmouth Citizens' Patriotic Recruiting Committee after the start of the First World War.
A formal remembrance service will take place at 11am, supported by local standard bearers and folk singer Simon Cattermole.
Chris Pennycook, of Pompey Pals, said: ‘The commemoration is held in September as our two battalions first went into a major action in September 1916 during the course of the Somme campaign.
‘The day will be a fitting tribute to our local service personnel from Portsmouth and the surrounding area who have served the country over the past 100 years.
‘The day will seek to commemorate, educate and help raise awareness of our local service veterans.’
The event has been organised with Ubique Right of the Line Living History Group and the Peter Ashley Activities Trust and will feature musical performances from the Portsmouth Military Wives Choir and TS Alacrity’s drummers.
Reenactors will wear period uniform spanning from the First World War to the 1980s and military vehicles and gear will go on display.
Organisers say Forgotten Veterans, Age Concern Portsmouth, Veterans Outreach Support, the Royal British Legion Riders Branch and the Commonwealth War Graves Commission will all also attend.
A vigil will also be held at the Cenotaph marking the 75th anniversaries of D-Day and the Battle of Arnhem, while an exhibition in Portsmouth Guildhall sheds light on artefacts from both World Wars.
Grenadier Guards will also be telling the story a Union flag that went to war and back from Bedhampton in 1945.