At the same time, the famed monument is also reopening its third story, closed since the 1970s, which offers a breathtaking view of Rome.
The two new attractions aim to boost visitor numbers at the site, which is already Italy's single most visited monument at some 19,000 people a day.
Crowds were already flocking to get up to the 33 meter (108 feet) third ring to enjoy its spectacular views Friday, while fans of ancient bloodletting will have to wait until Tuesday when groups of 25, strictly by reservation, will take the first steps by members of the public into the underground pits.
According to Colosseum site director Rossella Rea, the gladiatorial areas are all the more fascinating because "they were completely buried in the 5th century AD and have been perfectly conserved."
"They never suffered the depredation which the surface parts of the monument were victims to," ANSA quoted her as saying.
The so-called 'hypogeum' (literally, 'underground') has been restored in a project that has also installed new, muted lighting.
Rea said the aim was to recapture "some of the atmosphere" of the breathless moments before the games commenced, when the armored or naked fighters and the wild animals were hauled up through 80 trapdoors.
The visit starts from the Porta Libitinaria, named after the goddess of the dead Libitina, through which the gladiators marched in and from which their corpses were taken out, ANSA said.
Construction on the city's iconic monument started between 70 and 72 AD under the Emperor Vespasian.
It was completed in 80 AD by his son Titus. Titus inaugurated it with 100 days of games including the recreation of a sea battle between Romans and Greeks.