The demonstrations, which began in Istanbul on Wednesday, have since spread to more than 55 of Türkiye's 81 provinces, sparking violent clashes with riot police in the country's worst street protests in more than a decade.

Imamoglu's arrest came just days before he was to have been formally named as the main opposition CHP's candidate in the 2028 presidential race.

Following a night in which organisers said 300 000 protesters had rallied in Istanbul, there were similar numbers on Saturday. The boulevard outside the City Hall was a sea of red Turkish flags and angry banners reading: "Dictators are cowards!" 

On the fringes of the rally, protesters once again clashed with riot police, who fired tear gas, rubber bullets and pepper spray and also used percussion grenades, AFP correspondents said.

Meanwhile, about 10 kilometres away, around 1000 protesters gathered outside Caglayan courthouse where Imamoglu was being questioned by prosecutors investigating the claims against him, another correspondent said.

Outside, police had set up a tight security cordon with nearly 20 anti-riot vehicles at the ready, as protesters stood nearby, chanting: "Shoulder to shoulder against fascism!"

'Baseless and immoral'

Earlier Saturday, the 53-year-old mayor was quizzed by police for five hours. He denounced the accusations against him as "immoral and baseless", he said in a statement released by City Hall.

"This process has not only harmed Türkiye's international reputation but has also shattered the public's sense of justice and trust in the economy," he said. 

News of his arrest badly hurt the lira and caused chaos on Türkiye's financial markets with benchmark BIST 100 index closing Friday nearly 8.0 percent lower.

"We are here today to stand up for the candidate we voted for," 30-year-old Aykut Cenk told AFP outside the court, holding a Turkish flag.

"Just as people took the streets to stand up for Erdogan after the Jul 15 (2016) coup, we are now taking to the streets for Imamoglu," Cenk said. 

"We are not the enemy of the state, but what is happening is unlawful."

Journalists 'targeted'

The unrest has spread rapidly despite a protest ban in Türkiye's three largest cities and a warning from Erdogan that the authorities would not tolerate "street terror".

"For four days, they have been doing everything they can to disturb the peace and divide our people," railed Erdogan on Saturday. 

"The days when politics and justice are guided by street terror are totally in the past," he said. 

Earlier in the evening, Istanbul Governor Davut Gul said the authorities would not allow anyone to enter or leave the city who was "likely to participate in illegal activities". 

Police have arrested 343 people since the start of the protests, the interior ministry said on Friday. 

Türkiye's Journalists Union claimed the police had "deliberately targeted" journalists, saying many had been "severely beaten, shot with rubber bullets and had equipment broken". 

Reporters without Borders (RSF) also denounced the "heavy-handed and completely arbitrary" violence against journalists, demanding those responsible be "severely punished". 

Despite Imamoglu's detention, the CHP has vowed to press ahead with its primary on Sunday at which it would formally nominate him as the party's presidential candidate. 

It has pledged to open voting to anyone, not just party members, in the hope of garnering massive support for the beleaguered mayor, who is widely seen as the only politician capable of challenging Erdogan. 

Observers said the government would likely seek to block the vote.