Following a request by Turkish President Recep Tayyip Erdogan, opposition leader Kemal Kilicdaroglu is planning to take part in a demonstration this weekend against last month's attempted coup.
The head of the centre-left People's Republican Party (CHP) agreed to take part in the demonstration planned for Sunday in Istanbul, Turkish media reported.
The chief of the Nationalist Movement Party (MHP), Devlet Bahceli, had already said that he will take part in the event, alongside hundreds of thousands of other expected demonstrators.
The leader of the pro-Kurdish Peoples' Democratic Party (HDP) wasn't invited.
Erdogan has repeatedly accused the HDP, the nation's second-largest opposition party, of having links to the banned Kurdistan Workers' Party (PKK), though the HDP remains legal and denies that it answers to the armed group.
All three opposition parties represented in the Turkish parliament had previously condemned the July 15 putsch attempt.
The HDP was previously not invited to meet the president when he hosted the other opposition leaders following the failed coup attempt.