"Enough is enough," commented exasperated Tourism and Culture Minister Dario Franceschini on Twitter after visitors took to the microblogging site to vent their anger.
Assemblea al #Colosseo e turisti fuori in fila. La misura è colma:oggi in Consiglio Ministri proposta Musei come servizi pubblici essenziali— Dario Franceschini (@dariofrance) September 18, 2015
The incident follows similar no-notice shutdowns at Pompeii, another of Italy's most visited historical sites, over the course of the summer.
Adding to the confusion were posters put up at the Colosseum entrances which said the famous amphitheatre would reopen at 11:00 pm, when the union action was due to be over by 11:00 am.
"We didn't get told a thing," said Paule, a female French visitor from the Paris region. "We bought our tickets yesterday evening to get in first thing and we get here to find it all shut up."
All the other major archaeological sites in and around Rome were also closed as part of an ongoing dispute over security staff being required to manually check the bags of visitors, which they say makes their job impossible and leads to long delays in people getting in.
Bernabo Bocca, president of Italy's hotels and tourism association, slammed the unions involved.
"It is not he first time that the most attractive archaeological sites in our country have been closed without any warning to visitors from all over the world. We have to have a bit more respect for our tourists."
Some six million people visit the Colosseum, the biggest theatre built under the Roman empire, every year.