Anti-terrorism and bomb disposal experts from the Metropolitan Police and the Royal Navy have been called in after a walker heard what sounded like a bomb going off in Garadhban Forest, Loch Lomond.
When police officers arrived, they uncovered several explosive devices and found that a tree had been blown up.
Two areas of land were cordoned off to make the area safe. Shops and businesses in the nearby village of Gartocharn have been asked to hand over CCTV as a matter of urgency, and villagers and National Park staff have been told to report any suspicious activity they have seen in recent days.
Police have promised to continue intensive patrols in the area for the next three days, and a spokesman for the Scottish Government said ministers were being kept informed.
Officers refused to speculate on the motives for the explosion, but an expert said the signs all pointed towards an act of terrorism.