Excavation works at the Old Opera House site in Valletta to test its foundations in preparation for the Renzo Piano project started yesterday.
Workers dug up parts of the ruins to investigate the state of the foundations using heavy equipment.
The work had to be stopped because the vibrations dislodged a number of small stones that fell from a culvert into Yellow Garage, the parking area situated below the ruins.
No damage was caused, a government spokesman said.