By Ercan Gurses and Humeyra Pamuk
ANKARA/ISTANBUL (Reuters) - The main challenger for the leadership of Turkey's nationalist opposition vowed on Friday a party congress would go ahead this weekend despite the threat of police action, amid an internal party power struggle that could be crucial for President Tayyip Erdogan.
Several hundred members of the Nationalist Movement Party (MHP) have launched a bid to challenge Devlet Bahceli, its leader for much of the last two decades, hoping to change party rules at a special congress on Sunday in order to oust him.
Bahceli's removal could lead to a surge in support for the MHP, weakening the chances of Erdogan's ruling AK Party securing the stronger majority in parliament he wants for it to change the constitution and hand him greater powers.
Rallying around their veteran leader, MHP members loyal to Bahceli have challenged the legal basis for the special congress in several courts. A top appeals court decision is still pending, leaving the power struggle in legal limbo.
"The Ankara governor's office and the security forces have taken the necessary security measures, in line with written instructions from the relevant court, and will continue to do so," Yucel Bulut, an MHP lawyer, told a news conference, vowing any efforts to hold the congress would be blocked.
Bahceli's main challenger, Meral Aksener, a female 59-year-old former interior minister who polls suggest could double support for the MHP, vowed on Twitter to press ahead with the meeting, calling on party members to converge on an Ankara hotel congress hall on Sunday morning.
"I call on all our delegates. Our party congress this Sunday will go ahead," Sinan Ogan, another leadership challenger, told Haberturk TV, after a ruling by one of the courts involved paved the way for the meeting to go ahead.
But minutes later, state broadcaster TRT reported fresh verdicts from two other local courts ordering the meeting halted.
The AKP has increased its influence over the judiciary in recent years, and its opponents say the legal chaos surrounding the nationalists' congress is the result of clandestine efforts to keep Bahceli in power.
The AKP, founded by Erdogan, is seeking support from the MHP to hand him greater powers, something party members loyal to Bahceli are seen as potentially willing to do. Aksener has vowed to defend Turkey's current parliamentary system and has expressed her opposition to Erdogan taking more power.
AKP officials reject any suggestions that the government or ruling party is influencing the courts, or that the MHP's leadership battle and AKP efforts to win its support on constitutional change are in any way linked.
(Additional reporting by Orhan Coskun in Ankara; Writing by Nick Tattersall; Editing by Andrew Roche)