ANKARA, June 9 (Reuters) - The arch-rival leaders of Turkey's main opposition - one of them elected and the other controversially court-appointed - both announced plans to address party lawmakers at competing meetings on Tuesday, in a stand-offthat could deepen a crisis among President Tayyip Erdogan's challengers.

A Turkish court last month annulled the Republican People's Party's (CHP) 2023 congress that elected Ozgur Ozel as chairman, citing irregularities. It also reinstated to the post Kemal Kilicdaroglu, the party's divisive former leader who lost to Erdogan in the 2023 presidential election, in a move that hit financial markets and which critics described as politicised.

Both men had previously said they would address the weekly CHP meeting in parliament. MPs backing Ozel began gathering in the parliamentary hall several hours before it was due to begin at 1030 GMT. Kilicdaroglu then announced a new plan to hold a meeting at party headquarters, also in the capital Ankara.

"I am calling on all our party members, and every citizen whose heart beats for this country, to join us," Kilicdaroglu said in an X post, setting a time of 1100 GMT for his meeting.

The opposition's challenges could boost Erdogan's prospects of extending his more than two-decade rule of NATO-member Turkey in elections scheduled to be held by 2028, but which analysts say could come earlier if the government seeks to take advantage of the CHP strife.

Kilicdaroglu's return and recent criticism of the party have enragedhis detractors.

The meeting could mark one of Ozel and his elected team's last efforts to maintain control of the CHP, the party of modern Turkey's founder Mustafa Kemal Ataturk.

The secular and centrist CHP, running roughly even with Erdogan's Islamic-rooted and conservative ruling AK Party (AKP) in polls, has also faced an unprecedented judicial crackdown since 2024 in which hundreds of members and elected officials have been detained as part of corruption charges that the party denies.

Kilicdaroglu has said he would purge the party of corruption, referring to the cases involving CHP-run municipalities. The ousted leadership denies the allegations and calls them a politically motivated and anti-democratic "coup" - a charge the government rejects, saying courts are independent.

The CHP has 138 deputies in the 600-seat assembly and around two-thirds of them voted after the court ruling to make Ozel head of its parliamentary group.

Cavit Soydas, a CHP voter in the northeast village of Tekke, said on Sunday that Ozel "should fight through every possible legal means" to hang on to the party but if that fails "we are ready to go under the name of another party".

(Reporting by Huseyin Hayatsever and Ece Toksabay in Ankara; Additional reporting by Mirac Dereli in Tekke; Writing by Daren Butler; Editing by Jonathan Spicer, Alexandra Hudson)