The demonstrations erupted after the arrest of the mayor of Istanbul Ekrem Imamoglu, President Recep Tayyip Erdogan's main political rival, in a crackdown that has seen more than 1,400 people arrested, including 11 Turkish journalists.
Seven of them were remanded in custody by an Istanbul court on Tuesday, among them AFP photographer Yasin Akgul, drawing a sharp rebuke from the Paris-based news agency.
"His imprisonment is unacceptable. This is why I am asking you to intervene as quickly as possible to obtain the rapid release of our journalist," the agency's CEO and chairman Fabrice Fries said in a letter to the Turkish presidency.
The court charged Akgul, 35, and the others with "taking part in illegal rallies and marches", though Fries said Akgul was "not part of the protest" but only covering it as a journalist.
Media freedom group Reporters Without Borders (RSF) denounced the decision as "scandalous", with its Turkey representative Erol Onderoglu saying it "reflects a very serious situation in Turkey".
'Can't express ourselves freely'
Vast crowds have defied a protest ban to hit the streets daily since the March 19 move against Imamoglu, with the unrest spreading across Turkey and prompting nightly clashes with security forces.
On Sunday, Imamoglu was overwhelmingly chosen as the CHP's candidate for a 2028 presidential run, with observers saying it was the looming primary that triggered the move against him.
In the face of the biggest protests in Turkey since the 2013 Gezi uprising over the redevelopment of an Istanbul park, Erdogan has remained defiant, denouncing the rallies as "street terror".
"Those who spread terror in the streets and want to set fire to this country into a fire have nowhere to go. The path they have taken is a dead end," he said on Tuesday.
But as he spoke thousands of students marched through the Sisli district of Istanbul, whose mayor Resul Emrah Sahan was jailed in the same case as Imamoglu, heading for the district's municipal headquarters.
They chanted "government, resign!" and waving flags and banners with slogans including "Tayyip resign!", watched by a large deployment of riot police while people in apartments above bashed pots to show their approval.
Many had their faces covered with scarves or masks, and acknowledged they feared being identified by the police.
"We can't express ourselves freely," a student who gave her name as Nisa told AFP, saying she nonetheless joined the protest "to defend democracy".
In a separate protests, thousands also rallied for the seventh night in succession in a protest organised by Imamoglu's Republican People's Party (CHP) in the Sarachane district where Istanbul city hall that Immamoglu ran since 2019 is located.
Girding for what could be a long standoff, CHP leader Ozgur Ozel has called for a boycott of leading brands he says are linked to the authorities including coffee chain Espressolab and retailer D&R.
'Dark time for democracy'
With riot police using water cannon, pepper spray and rubber bullets against protesters, the Council of Europe denounced the "disproportionate" use of force while Human Rights Watch said it was a "dark time for democracy" in Turkey.
The United Nations also voiced alarm at Turkey's use of mass detentions and its "unlawful blanket ban on protests", urging the authorities to probe any unlawful use of force.
"All those detained for the legitimate exercise of their rights must be released immediately and unconditionally," UN rights office spokeswoman Liz Throssell said.
Imamoglu, 53, is widely seen as the only politician capable of defeating Erdogan, who has ruled the NATO member for a quarter of a century.
But Imamoglu has now been stripped of his mayorship and jailed over a graft and terror probe that his supporters denounce as a "political coup".
By Tuesday, police had detained 1,418 suspects for taking part in "illegal demonstrations", Interior Minister Ali Yerlikaya wrote on X, warning there would be "no concessions" for those who "terrorise the streets".
But Ozel called for a mass rally at the weekend to protest the arrest of Istanbul mayor, demand open trials and call for early elections.
"Are you ready for a big rally in a large square in Istanbul on Saturday to support Imamoglu, to object to his arrest, to demand transparent, open trials, to say we have had enough and we want early elections?" Ozgur Ozel asked protesters in a speech, telling them the rally would be held in the vast Maltepe grounds on the Asian side of Istanbul.
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