MADRID -- Crowds gathered Tuesday at Madrid's Prado Museum to view a copy of the "Mona Lisa" for the first time since restoration revealed it was almost certainly painted by one of Leonardo da Vinci's apprentices as he worked on the original.
The painting is on display until March 13, after which it will move to Paris' Louvre museum to hang alongside the original as part of an exhibition on da Vinci's work.
Although the precise author of the copy has not been determined, both the Prado and Louvre believe it is probably the earliest known copy of "La Gioconda," as the painting is also known since it is believed to depict Lisa, the wife of Francesco del Giocondo.
The Prado described it as "the most important version known to date of Leonardo's emblematic painting."
The copy has been part of Spain's art collection for hundreds of years. It had previously been on display in the Prado but was given little attention as it was considered a mediocre copy.
But hours after opening Tuesday, dozens of people packed the area around the work to try to get a closer look, triggering further comparisons with the original in the French capital.
"It's a good thing for the Prado – anything that brings people here and encourages them to look and to think," said Gabriele Finaldi, the museum's deputy director of conservation and research.
The Prado said it will continue to hang the copy alongside other early Italian works when it returns from Paris, but consider moving it to a more spacious area if it continues to attract big crowds.
"This, after all, is a copy, said Finaldi. "The interesting thing is that it's a copy that takes you into Leonardo's studio. It's not a copy made afterward. It's made at the same time as the great master is working."
Visitors were uncertain what to think.
"I think the Mona Lisa that we have always known is much more romantic, maybe because I thought of it as unique," said Spanish nurse Marta Diaz. "The woman in the copy is much prettier but I prefer the original."
For more than 200 years the background behind the Gioconda copy was black, not the pretty mountainous landscape of the "Mona Lisa".
But when work began two years ago to get it ready for the da Vinci exhibit, X-ray tests revealed the landscape underneath and showed that changes made in the copy were similar to those made to the original as it evolved.
"It was a genuine revelation," said Finaldi.
Blackening backgrounds to highlight figures was not an unusual practice with paintings in the past.
The Prado hopes the work will help further studies into the "Mona Lisa," and says it reveals details and sketch lines no longer immediately apparent in the original.
Varnish has been removed from the face of the copy, making it brighter and younger than the face of the "Mona Lisa" coated with cracked, darkish varnish at the Paris museum.
Finaldi said part of the veil and braid are much clearer in the copy, having been obscured by varnish applied to the "Mona Lisa".
The discovery is likely to further fuel debate over whether the Louvre's "Mona Lisa" should be subject to further restoration or continue to be left alone.
One major difference from the original is that the figure in the copy has eyebrows and the Mona Lisa in the real masterpiece does not.
There are dozens of the surviving replicas of the masterpiece from the 16th and 17th centuries.