Hundreds of thousands of people have gathered in Istanbul to protest in defence of the city's opposition mayor Ekrem Imamoglu, defying President Recep Tayyip Erdogan's warning that Turkey will not tolerate "street terrorism".

It was the third night in a row that demonstrators rallied against the arrest of Imamoglu - Erdogan's biggest political rival - in Turkey's largest protests in more than a decade.

Opposition leader Ozgur Ozel, head of the Republican People's Party, which called the nationwide protests, told the huge crowd outside Istanbul's City Hall that 300,000 people had joined the demonstration.

"This is not an ANP rally, the people here are from all parties and have come to show solidarity with Mayor Imamoglu and to stand up for democracy," he told people who interrupted his speech with applause.

Erdogan "is trying to twist Imamoglu's arms, is using the judiciary as a weapon and wants to take over this building. But we will not hand it over to a government-appointed trustee!" he said.

Riot police used tear gas and rubber bullets against the protest.

Clashes also erupted in Ankara and the western coastal city of Izmir, where police used water cannons and tear gas against people.

"Do not be silent or you will be next", demonstrators in Istanbul chanted while waving placards reading "Do not be afraid, the people are here!" and "Law, rights, justice".

Imamoglu's arrest came just days before he was officially announced as the RNP's candidate in the 2028 presidential race.

The protests spread rapidly from Istanbul to at least 40 of Turkey's 81 provinces. | BGNES, AFP