ISTANBUL (Reuters) -Turkish authorities ordered more detentions of opposition officials on Wednesday, in a fresh round of legal manoeuvresagainst President Tayyip Erdogan's opponents that has prompted billions of dollars in foreign reserve sales.

The months-long crackdown has swept up hundreds of members of the Republican People's Party (CHP), including Istanbul Mayor Ekrem Imamoglu - Erdogan's main political rival - whose March arrest set off Turkey's largest street protests in a decade.

The latest moves against the CHP began on Tuesday when a court ordered the removal of the party's Istanbul provincial head over alleged irregularities in a 2023 congress.

That sparked a selloff of Turkish stocks and bonds and galvanised the central bank into action. According to traders, the bank sold $4 billion to $5 billion in reserves to stabilise the lira, which has held mostly steady over the last two days.

The central bank declined to comment.

The CHP, which is running neck-and-neck with Erdogan's AK Party in opinion polls, said the party official's removal was illegal.

And with a separate looming court decision threatening to remove the CHP's national party chief, a European Parliament diplomat warned of threats to the country's multi-party democracy.

State broadcaster TRT said six officials from two opposition-run municipalities in Istanbul were detained and another was being sought on Wednesday.

The mayors of those two districts are among 15 CHP mayors jailed as part of the sprawling investigation that began late last year into alleged corruption and other offences in exclusively opposition-run municipalities.

The CHP denies the accusations, saying the legal moves are politicised.

NAIL IN DEMOCRACY'S COFFIN

Justice Minister Yilmaz Tunc has said the judiciary is independent and called the CHP's criticism of the court's removal of its Istanbul party boss unfortunate.

He also said that court decision may weigh on a separate but similar ruling due on September 15 in the capital Ankara that will decide whether to annul the CHP's main 2023 congress and strip party leader Ozgur Ozel of this chairmanship.

The Ankara court has also been reviewing alleged procedural irregularities.

Nacho Sanchez Amor, the European Parliament's Turkey rapporteur, said Tuesday's ruling appeared aimed at preparing the way for Ozel's removal.

"That would amount to the nail in the coffin of Turkish multiparty democracy," he wrote on X.

Shares in Istanbul's main BIST 100 were down 2% on Wednesday after having fallen as much as 5% on Tuesday.

Ozel said the party would object to the Istanbul ruling in every way possible on the grounds that the court had usurped the authority of Supreme Election Board, which he said had sole jurisdiction over such matters according to the constitution.

"If a court of first instance is able to rule on matters under election law, then any court may annul any election, including presidential elections," he said in front of CHP's Istanbul headquarters.

CHP supporters and anti-Erdogan activists are expected to gather later on Wednesday when Ozel addresses a rally in Istanbul.