Thousands gather near Istanbul’s town hall in support of the city’s recently arrested mayor.

Thousands of protesters have joined a march in Istanbul in support of the city’s arrested mayor, defying a warning from Turkish President Recep Tayyip Erdogan that authorities would crack down on “street terror”.

Demonstrators took to the streets in Turkiye’s commercial hub on Friday for the third consecutive day to show their support for Ekrem Imamoglu – Erdogan’s chief political rival – who was arrested on Wednesday, days before he was due to announce his 2028 presidential run.

Opposition leader Ozgur Ozel said more than 300,000 people had joined protests across Istanbul.

“We are 300,000 people,” the CHP leader told the vast crowds in front of City Hall, saying protesters had gathered at several places across the country’s largest city due to the road and bridge closures preventing people from all being in one place.

The mayor was arrested early on Wednesday over alleged corruption and “terror” links. Dozens of other prominent figures, including journalists and businesspeople, were also detained. After the detentions, the government announced a four-day ban on political demonstrations.

The arrests came a day after a university in Istanbul invalidated Imamoglu’s diploma, in effect disqualifying him from the presidential race because having a university degree is required to run for the nation’s highest office under the Turkish Constitution.

Imamoglu said he would challenge Istanbul University’s decision.

Erdogan on Friday said the government would not tolerate street protests and accused Imamoglu’s Republican People’s Party (CHP) of links to corruption and “terror” organisations.

“An anticorruption operation in Istanbul is being used as an excuse to stir unrest in our streets. I want it to be known that we will not allow a handful of opportunists to bring unrest to Turkiye just to protect their plundering schemes,” Erdogan said.

Government critics viewed Imamoglu’s arrest as an attempt to remove a key challenger to Erdogan from Turkiye’s next national ballot.

Government officials rejected accusations that legal actions against opposition figures are politically motivated and insisted that Turkiye’s courts operate independently.

Reporting from Istanbul, Al Jazeera’s Aksel Zaimovic said there were “many university students” at the protest outside Istanbul’s municipality building.

“They say they are protesting the arrest of Imamoglu as well as the decision by Istanbul University to revoke Imamoglu’s diploma,” he said.

Zaimovic also noted that protesters told him “this movement is intended to send a message about ‘systemic injustice’ in Turkish society rather than supporting any one political party.”

“Many say their right to elect an individual to represent them is being challenged by these latest developments,” he added.

Protests over Imamoglu’s arrest began in Istanbul on Wednesday and quickly spread to 32 of Turkiye’s 81 provinces, according to an AFP news agency count.

The CHP, Turkiye’s main opposition party, which was intending to make Imamoglu its presidential candidate on Sunday, has described his arrest as a “coup”.

“Don’t be silent! Otherwise, they’ll come for you,” protesters yelled. Many held aloft placards with slogans such as “Don’t be afraid, the people are here” and “Rights, law, justice”.

Since the protests erupted, at least 88 protesters have been arrested, Turkish media reported, with Interior Minister Ali Yerlikaya saying 16 police officers had been hurt.

Police have also detained another 54 people for online posts authorities deemed “incitement to hatred”, he said.

Imamoglu was elected mayor of Turkiye’s largest city in March 2019 in a historic blow to Erdogan and the president’s Justice and Development Party, which had controlled Istanbul for a quarter-century.