A massive Istanbul demonstration over the arrest and jailing of opposition presidential candidate Ekrem Imamoglu and the fate of democracy in Turkey is planned for Saturday (March 29).

There will be concern that the event could prove a fateful test of the willpower of the opposition and the movement to free Imamoglu and the ruling regime.

Tuesday night brought the last of seven nights of mass protests held outside Istanbul municipal headquarters called by the main opposition Republican People’s Party (CHP). They have served as the centrepiece of nationwide demonstrations that have brought hundreds of thousands to the streets nationwide, though Turkish public television and much of the rest of the country's media have ignored them.

Ozgur Ozel, head of the CHP, told the BBC on March 26 that protests would continue "in every city" until either a snap presidential election was called, or Imamoglu, who was stripped of his position as mayor of Istanbul by the interior ministry after his detention on corruption and terror allegations, was released from prison.

The Saturday demonstration in Turkey’s commercial and cultural capital would also open his party's campaign to make Imamoglu the country's next president in either a an early election or the election that is due in 2028 at the latest, he said.

"In every city we go to, we will have the biggest rallies in their history," Ozel declared. "The belief in Ekrem Imamoglu and in democracy will make the protests bigger and stronger."

The protest crowds, which form despite a ban on such gatherings announced after the Imamoglu arrest, are the biggest seen in Turkey in over a decade. More than 1,400 people including many journalists have been arrested.

The Turkish police's response to the protests has included assault, the liberal use of tear-gassing and the firing of plastic bullets.

Imamoglu and the CHP say the allegations and charges brought against Imamoglu, seen as a politician with a tremendous chance of unseating Turkish leader of 22 years Erdogan in a fair contest, are entirely politically motivated and designed to remove him from the political fray. The Erdogan administration rejects that accusation, saying prosecutors and the courts are independent. But the fact the detention came days before Imamoglu was due to be officially made a presidential contender does not help their case.

Turkey's President Recep Tayyip Erdogan has taken to calling the demonstrations "street terrorism". The opposition's "show" would eventually fade, he has said, while accusing the protesters of wrecking Turkey's economy. Erdogan himself has long stood accused of wrecking Turkey's economy.

Imamoglu is being held at the infamous Silivri Prison, a high-security facility on the outskirts of Istanbul.

"He is in solitary confinement, but he's in good condition and has not been mistreated so far," he told the BBC.

"Erdogan has thrown a three-time [Istanbul mayorship] election winner in jail… in front of the whole world," Ozel told the British broadcaster.

"Suddenly he is jailing someone who is fighting against him in a normal political way. It's like your rival coming and slicing the ball in a football game, because you are winning."