Thousands of tourists have been kept out of the Colosseum by a union meeting, prompting the culture minister to vow measures to make sure it doesn't happen again.

Dario Franceschini expressed anger that for an entire morning on Friday, "tourists were left to wait in line in front of the whole world, this time at the Colosseum, the most important archaeological site in Rome."

He said Premier Matteo Renzi agreed to pursue measures to protect Italy's cultural treasures from strikes, with new norms defining Italian monuments and museums as essential services.

It's not the first time that a labor dispute left tourists waiting. A strike at Pompeii earlier this summer left lines of tourists baking in the sun outside the World Heritage archaeological site.