Ekrem Imamoglu, the mayor of Istanbul, was detained on Wednesday and formally arrested and jailed pending trial over corruption charges earlier on Sunday.
Police used pepper spray and fired rubber bullets into the crowd that gathered outside Istanbul’s city hall.
Reporting from Istanbul on Sunday evening, Sky News’ special correspondent Alex Crawford described the protests as “relatively peaceful” but said they were “the biggest protests Turkey has seen in more than a decade”.
The huge demonstrations have continued despite a ban on gatherings in Istanbul imposed this week – but many protesters hid their faces with masks.
Most protesters did not want to speak on camera as they were scared of being recognised, Crawford said, but they took to the streets because “they feel their democracy is being eroded”.
She added questions remained over how long authorities would allow these protests to continue – and whether they will escalate into a real crisis for President Recep Tayyip Erdogan.
The protests intensified after Imamoglu was charged on Sunday with running a criminal organisation, accepting bribes, extortion, illegally recording personal data and bid-rigging. A request to detain Imamoglu on terror-related charges was denied.
Following the ruling, he was transferred to Silivri prison, west of Istanbul.
Imamoglu has labelled all of the claims “unimaginable accusations and slanders”.
Many view his detention as a political move to remove him from the 2028 presidential race – deepening concerns over the country’s democracy and the rule of law.
The Turkish government denies Imamoglu’s arrest was politically motivated and says the courts are independent.
Erdogan has served as Turkey’s president since his Justice and Development Party (AKP) came to power in 2003.
Its biggest opponent in the next election looks set to be the Republican People’s Party (CHP), which this weekend began holding a primary presidential election to endorse Imamoglu, their only presidential candidate.
The party also set up solidarity boxes where non-members could show their support – and at the time of last night’s protest, more than 13 million had done so, according to the CHP.
In a post on social media, Imamoglu praised the result, saying the people had told Mr Erdogan: “Enough is enough.”
“That ballot box will arrive and the nation will deliver a slap to the administration it will never forget,” he added.
Imamoglu – who could be prevented from running for president if he is convicted – was one of 48 people jailed pending trial this weekend. The others being held include a key aide and two Istanbul district mayors.
While the protests have largely been peaceful, there has been some violence in Istanbul, as well as other cities including Ankara and Izmir.
Clashes have seen police deploy water cannon, tear gas and pepper spray at protesters, some of whom threw stones and fireworks at riot officers.
Turkish Interior Minister Ali Yerlikaya also announced on Monday that 1,133 people have been arrested since protests against the arrest of Imamoglu began last week.
“The terrorization of our streets and the threat to the peace and security of our nation will absolutely not be tolerated,” Yerlikaya wrote on X.
Close to 15 million people voted for Imamoglu in a presidential primary organised by the CHP, municipal officials has announced.
In a sign of growing national support, the party announced late on Monday that an estimated 1.7 million members were joined by 13 million non-party members in supporting Imamoglu to become its official candidate for the next presidential election, which is due to take place in 2028.
