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Turkish stocks tumbled after a court said the appointment of the Istanbul head of the main opposition party was invalid, a potentially landmark ruling that could hobble challengers to President Recep Tayyip Erdoğan.

The benchmark BIST 100 index dropped almost 6 per cent following the court decision, before closing more than 3.5 per cent lower.

The Istanbul court ruled that voting in the 2023 Istanbul provincial congress of the Republican People’s party (CHP) was influenced by cash payments, and, therefore, the leadership elected at the congress — including provincial chief Özgür Çelik — should be removed.

The market move recalled the investor panic that followed the arrest on corruption charges in March of Istanbul mayor and CHP presidential candidate Ekrem İmamoğlu.

Dozens of opposition mayors and officials have since been arrested in a crackdown on the opposition that has, in effect, removed the government’s biggest rivals.

The government insists that Turkey’s courts are independent, but the CHP says this and other cases against it are politically motivated.

“The Republican People’s Party . . . cannot be taken over!” Çelik wrote in a social media post, adding that his phone had been locked and he could not respond to messages or calls.

The court appointed former CHP deputy chair Gürsel Tekin as the interim chief in Çelik’s stead.

Justice minister Yılmaz Tunç said the court’s decision was a “precautionary” measure and did not “constitute a final judgment”.

Nevertheless, Tuesday’s ruling may pave the way for another adverse court judgment against the CHP later this month, which could lead to the removal of CHP chair Özgür Özel.

That lawsuit, which is due to be heard on September 15, alleges that the results of a CHP congress that elected Özel as its chief in 2023 could also be overturned due to procedural irregularities.

Such a ruling could reinstate Kemal Kılıçdaroğlu, the party’s former leader whose tenure was widely criticised and who lost the 2023 presidential election to Erdoğan.

That court case is also just one of several against the CHP that are due to be heard this autumn, including against İmamoğlu.

“Today, Turkish courts are delivering verdicts that move the country rapidly from ‘electoral authoritarianism’ to something more akin to outright dictatorship,” Howard Eissenstat, non-resident scholar at the Institute for Turkish Studies at Stockholm University, wrote in a social media post.

“Elections remain, but the last vestiges of competitive politics [are] being erased.”

The crackdown comes as polls have shown that the CHP would beat Erdoğan’s governing Justice and Development Party were an election held today.

Several large cities including Istanbul — which accounts for a third of Turkey’s GDP — have recently shifted to CHP control and the opposition made further gains in last year’s local elections.

İmamoğlu’s arrest in March triggered mass protests and an investor exodus that prompted the central bank to jack up interest rates and spend about $50bn to support the currency.

Foreign reserves have since partially recovered but Turkish assets have remained highly sensitive to political risks.

“These moves . . . form part of Erdoğan’s broader strategy to disable the main opposition party and weaken the opposition ahead of the next presidential and parliamentary elections,” Wolfango Piccoli, co-president of Teneo, a consultancy, wrote in a note to clients.