The three chunks of mortar plummeted to the ground around dawn on Sunday, a few hours before thousands of tourists tramped through the gladiatorial arena.
They crashed through a wire protection net which was supposed to have prevented such accidents, but which is more than 30 years old.
Archeologists warned that disaster had only narrowly been averted and that visitors could have been badly injured or even killed by the debris.
The plaster, which dates from Roman times, fell from a 10 square foot section of roof in one of the stone entrance ways through which spectators used to file to watch gladiators take on wild animals, prisoners-of-war and each other.
A 23 million euro restoration and cleaning project is meant to get under way in the next few weeks, but Rome city council is still trying to raise funds from the private sector in Italy and abroad to finance the work.
Authorities said the loosening of the plaster may have been caused by recent heavy rain, humidity and temperature changes.
Archaeologists said the near miss should act as a wake-up call for the parlous state of the arena, which was started by Emperor Vespasian in 72AD and subsequently suffered damage from earthquakes and centuries of pillaging.
“Once again we’ve come close to tragedy,” said Giorgia Leoni, the president of the Association of Italian Archeologists.
“If the collapse had happened during opening hours, it could have hit one of the thousands of visitors who, especially on Sundays, crowd into the Colosseum.” Attempts in the past to strengthen the Colosseum had been woefully inadequate, said Andrea Carandini, the president of the Council for Culture and Heritage.
“Decades and decades were lost,” he said. “The problems of the Colosseum and of other ancient monuments in Rome were never taken by the horns. Sooner or later something was going to happen.” The Colosseum, which attracts 3.2 million visitors a year, is not the only ancient monument in Rome that is crumbling.
In March this year, the roof of the Domus Aurea, a magnificent palace built by Emperor Nero but covered up by succeeding emperors, caved in, damaging the interior.
In 2001, a section of the Aurelian Wall, the stone ramparts which were built to defend Rome from barbarian invasion in the 3rd century AD, collapsed as a result of heavy rain and subsidence.