Market, Church and Arwenack Streets are closed to traffic for the four days of the tall ships regatta and Fal FoE will be marking the "all too brief" road closure.
Members will be joined by Trystan the tortoise, who is the size of a 4X4, but "more climate friendly."
Trystan will stroll up and down the town centre, indulge in a little "danger-free" window shopping, chat to friends and listen to music.
He will invite those he encounters to join him and Fal FoE in a "celebration of slowness."
Fal FoE member Tom Scott said: "Motor cars given free reign to charge along the main shopping streets is very much the same plain silliness as overtook the hare in Aesop's famous fable and even more so when this four days so vividly demonstrates the feasibility of doing without a car-infested shopping centre or Church Street car park."
Fal FoE has staged a series of events to draw attention to their campaign for a car-free town centre. The group now awaits the possible installation of a rising bollard, which will restrict traffic through the centre.
Fal FoE member Betty Levene said: "The bollard is a really good step in the right direction, but only that.
"Shoppers, holiday-makers and town centre workers will still have to endure the same perils, discomfort and fumes along most of the main streets. So, it's really important that this is seen as a first initiative and that no-one settles for this much less than optimal option."