Kemal Kilicdaroglu nominated his candidacy for the head of Turkish Republican People's Party.
Former party chairman Deniz Baykal was forced to resign after the video of his sexual personal life appreared in the Internet.
Besides Kilicdaroglu, MP Haluk Koc, businessman Umut Oran and academic Suheyl Batum claimed this post.
Speaking in opposition to the policy of the ruling Justice and Development Party in the general elections of 2007, the People's Republic Party formed a coalition with the Democratic Left Party. The coalition received 22 percent of voters. About 112 members became MPs.
The final vote to elect a new leader will be held May 22.