Prime Minister Matteo Renzi said the completion of the first part of the project was a "sensational signal to the world", at a ceremony marking the event, also attended by Culture Minister Dario Franceschini and Italian luxury entrepreneur Diego della Valle.
Renzi added that the first phase of the renovation had ended slightly ahead of schedule.
The world-famous monument built in the Italian capital between 79-80 AD has been undergoing a 25-million-euro renovation since 2013, funded by Della Valle's shoe giant Tod's.
The second phase of the renovation, also funded by Della Valle, will concern the Hypogeum (underground) area of the Colosseum.
At the ceremony on Friday, Franceschini said work to give the Colosseum a floor so that its interior regains the original appearance should be completed by 2018.
"The work for the arena should be completed within three years (of the allocation of funding) and therefore in 2018," Franceschini said.
The minister said that giving the Colosseum a floor will enable it to become the venue for cultural events, including concerts and shows.
It is currently not possible to walk around the middle of the Colosseum as its network of underground tunnels and pits is exposed.
ANSA
