ISTANBUL (Reuters) -Thousands of Turks ramped up protests on Thursday despite a ban on street gatherings over what they called the undemocratic detention of Istanbul's mayor Ekrem Imamoglu, as the opposition sought to pin the blame on President Tayyip Erdogan.

Even as police erected barricades and as dozens of people were detained over social media posts, protesters gathered at a municipal headquarters and police station in Istanbul, and at universities and public squares across the country.

Imamoglu, 54, Erdogan's main political rival, was taken in on Wednesday facing charges of graft and aiding a terrorist group, a move that the opposition condemned as a "coup attempt" and that sparked an initial round of demonstrations and criticism from European leaders.

The move against the popular two-term mayor caps a months-long legal crackdown on opposition figures that has been criticised as a politicised attempt to hurt their electoral prospects and silence dissent, charges the government denies.

In an interview, Ozgur Ozel, chairman of the main opposition Republican People's Party (CHP) that includes Imamoglu, said Erdogan feared facing the mayor in a vote and wanted to "take him out of the game" and cut his party's ties with the city.

He said that any move barring Imamoglu from running for president in the next election would only strengthen the opposition's support, adding the CHP will appoint him its candidate on Sunday at a scheduled vote.

"We believe Imamoglu will win. If his candidacy is blocked, we believe this will turn into much greater support," Ozel told Reuters in his first interview with foreign media since the detention.

"This nation has never forgiven anyone trying to lift the legitimacy of the ballots," he said next to a small room at the Istanbul municipality headquarters - with crowds gathering outside - where he intends to overnight until Imamoglu is released.

WARNINGS

Elections are not scheduled until 2028 but would need to come earlier if Erdogan, 71, who has run Turkey for 22 years, wants to run again. Imamoglu leads the president in some polls.

The government rejects the criticism and says the judiciary is independent. It has warned against tying Erdogan or politics to Imamoglu's arrest, after which it imposed a four-day ban on gatherings and restricted access to some social media to restrict communications.

Police blocked off roads and stationed trucks with water cannons near the police station where the mayor is held and other areas of Turkey's largest city.

Imamoglu on Thursday called on members of the judiciary and Erdogan's ruling party to fight the injustice of his detention.

"These events have gone beyond our parties or political ideals," he said on X. "It is time to raise our voices."

Civil disobedience has been dramatically curbed in Turkey since the nationwide Gezi Park protests against Erdogan's government in 2013, which prompted a violent state crackdown.

But crowds in Istanbul, Ankara and other cities have been chanting anti-government slogans and, in Istanbul, they hung banners of Imamoglu and the nation's founding leader Mustafa Kemal Ataturk alongside Turkish flags.

"They hastily detained our mayor, whom we elected with our votes," said Ali Izar, an opposition supporter in central Istanbul. "I do not think this is a democratic practice and I condemn it."

MARKET FALL

The detention sparked a crash and partial recovery in the lira on Wednesday, which by Thursday was worth nearly 38 to the dollar, compared to 36.67 beforehand. Amid worries about eroding rule of law and concerns over slower rate cuts, bank shares in particular tumbled on the Istanbul bourse, and the central bank hiked its overnight rate.

Interior Minister Ali Yerlikaya said that 37 people had been detained and accused of "provocative social media posts inciting crime and hatred" after the detention.

Authorities also seized a construction company co-owned by Imamoglu and handed control over to a court, according to the Istanbul Chief Public Prosecutor's office and financial crime investigation reports.

Istanbul municipality staff made up the bulk of the 105 people arrested along with Imamoglu.

(Additional reporting by Mert Ozkan and Mehmet Emin Caliskan in Istanbul and Tuvan Gumrukcu and Huseyin Hayatsever in Ankara; Writing by Jonathan Spicer; Editing by Alex Richardson, Andrew Cawthorne, Alexandra Hudson)