Speaking at a news conference, senior party member Giorgy Gugava said: "the current regime must be changed to save the country."
President Saakashvili has ruled out early presidential or parliamentary elections, saying that 2008 saw early presidential, parliamentary and local elections. The president's second term expires in 2013. However, the opposition is demanding early elections, and plans to hold anti-presidential street rallies on April 9.
Natelashvili is not the first challenger to Saakashvili's leadership. Late last month, an alliance of opposition groups put forward their candidate, Irakly Alasaniya, a former UN ambassador and one of the most popular politicians in the country with strong ties with the U.S.
The president's former ally and parliament speaker, Nino Burdzhanadze, has also announced her candidacy.
Alasania earlier issued an ultimatum to the president, saying he should schedule a referendum to decide on early elections by March 5, to avoid mass protests.
Gugava said the party was launching discussions with other opposition groups aiming to join efforts to topple the Saakashvili regime, form a coalition government, outline new foreign policies and amend the constitution.
Georgia's opposition has demanded Saakashvili's resignation, most recently blaming him for the August war with Russia over South Ossetia.
