The phosphorous grenades were found opposite Pizza Hut on the northbound carriageway of Tavistock Road.
Nearby homes and businesses were evacuated and the route closed in both directions with a 100m (300ft) cordon in place.
The Royal Navy's Explosive Ordnance Disposal team used a controlled explosion to make the grenades safe.
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One of the main roads in Plymouth was closed after eight unexploded grenades were discovered by workers.