A Turkish court on Sunday jailed Istanbul Mayor Ekrem Imamoglu, President Tayyip Erdogan's main political rival, pending trial on corruption charges, in a move likely to inflame the country's biggest protests in more than a decade.

The decision to send Imamoglu to jail comes after the main opposition party, European leaders, and tens of thousands of protesters criticized the actions against him as politicized and undemocratic.

As the courtroom developments unfolded, there were signs that the mayor's troubles were galvanizing opposition against Erdogan's government, which has run Turkey for 22 years.

Thousands of Republican People's Party (CHP) members and non-members streamed into polling stations nationwide to elect Imamoglu, its candidate, in a future presidential vote.

The non-member vote will be closely watched as an indicator of how much support the widely popular Imamoglu enjoys beyond the party faithful.

Imamoglu has denied the charges he faces as "unimaginable accusations and slanders" and called for nationwide protests on Sunday. "We will rip away this coup, this dark stain on our democracy, all together," he said.

Footage showed what broadcasters said was that he was being taken to Silivri prison in a police convoy.

Imamoglu was removed from duty, along with two other district mayors, according to an interior ministry statement.

The government denies that investigations are politically motivated and says courts are independent.

A nationwide ban on street gatherings was extended on Saturday for four more days but protests and skirmishes with police continued through the night in major cities.

Thousands crowded outside the courthouse overnight and early on Sunday, awaiting the rulings on Imamoglu.

The court said Imamoglu, 54, and at least 20 others were jailed as part of a corruption investigation, one of two opened against him last week.

It said he was arrested for "establishing and leading a criminal organization, accepting bribes, embezzlement, unlawfully recording personal data, and rigging public tenders in connection with a financial investigation".

The jailing caps a months-long legal crackdown on opposition figures and the removal of other elected officials from office, in what critics called a government attempt to hurt their election prospects.

Six of the CHP's 27 municipal mayors are now under arrest - a year after opposition parties handed Erdogan's AK Party its worst-ever electoral defeat in municipal elections.

The CHP opened party polling stations on Sundays for non-members to cast "solidarity votes" for Imamoglu, who was the only name on the ballot for presidential candidate.

CHP Chairman Ozgur Ozel predicted millions of votes from non-members, saying Imamoglu is "on the road to prison but also to the presidency".

No general election is scheduled until 2028.

But if Erdogan, 71, who has led Turkey for 22 years, is to run again, parliament would need to back an earlier election since the president will have reached his limit by that date. Imamoglu is leading Erdogan in some opinion polls.

Imamoglu is also facing terrorism charges, but the court did not formally arrest him on those at the same time.

A future ruling to jail him pending trial on terrorism-related charges could allow the government to appoint a trustee to run Istanbul. A conviction could prevent him running for president.

The CHP said it would appeal against the ruling and elect someone to work as acting mayor.

Shortly after the court ruling, the mayor vowed to ultimately defeat Erdogan, and said those who ran the investigation would be held accountable.

"Imamoglu has become Erdogan's ... nightmare," Mehmet Karatas, an opposition supporter, said outside the courthouse. "We will make Ekrem Imamoglu president."

Imamoglu's detention has roiled markets, with the Turkish lira, stocks and bonds suffering heavy declines since Wednesday. Analysts expect more selling pressure after the jailing.

Turkish central bank governor Fatih Karahan met board members of Turkey's Banks Association (TBB) on Sunday.

The central bank will use all instruments within market rules effectively and decisively to maintain stability, Karahan told bankers, the TBB said.

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

Yet thousands gathered again Saturday night outside the Istanbul municipality building and the main courthouse. Hundreds of police used tear gas and pepper spray to disperse protesters, as the crowd hurled firecrackers and other objects.

Though most demonstrations have been peaceful, protesters also clashed with police in the western coastal province of Izmir and the capital Ankara for a third night in a row, with police firing water cannon.

Authorities have detained more than 300 people during protests.