The head of Turkey's main opposition party has told the BBC that protests will continue "in every city" until either early presidential elections are called or the jailed mayor of Istanbul, Ekrem Imamoglu, is released from prison.

Ozgur Ozel, chairman of the mayor's Republican People's Party (CHP), said the nationwide protests would include a very large demonstration this Saturday in Istanbul. That will open the party's campaign to make Imamoglu the country's next president in elections that are due in 2028, 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," he told us at his party headquarters in Istanbul, as visitors, staff and advisors bustled in and out.

The opposition has brought huge crowds onto the streets - the biggest seen here in over a decade - since Imamoglu was arrested seven days ago.

Alongside the mass demonstrations, there have also been mass arrests - more than 1,400 people and counting, including seven Turkish journalists who were reporting on the protests.

Turkey's President Recep Tayyip Erdogan, who has plenty of strong supporters, has condemned the demonstrations as "street terrorism" and accused protesters of attacking the police and damaging public property.

He said the opposition's "show" would eventually fade.

Ozel spoke to the BBC fresh from a visit to Silivri Prison, a high-security campus on the outskirts of Istanbul where Imamoglu is being held.

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

Ozel said the corruption case against Istanbul's mayor was "a scam designed to discredit him".

As an example, he cited allegations that Imamoglu bought land cheaply years ago, and the low purchase price may have been a bribe. "The truth was that small payment was just the deposit for the land," he said.

Imamoglu denies all the charges against him, including "establishing a criminal organisation, taking bribes, extortion, and rigging a public tender".

He says his arrest was a coup. Turkish officials say the courts here are independent. Human rights organisations strongly dispute that.

Ozel said Imamoglu was arrested for one simple reason – to prevent him becoming Turkey's next president. Opinion polls suggest the mayor might be able to do that – if he's not behind bars.

"Erdogan has thrown a three-time election winner in jail… in front of the whole world," Ozel said.

"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."

The opposition party believes the response of Turkish society and the international community will be key in deciding if Imamoglu remains behind bars.

But Ozel said the CHP felt "abandoned" by the UK Prime Minister Keir Starmer and his party.

"While all of Europe is reacting, the English Labour Party, and Starmer have said nothing. The cradle of democracy - England - and our brother party, the Labour party, how can they stay silent? We are really hurt."

A few hours before that comment on Tuesday, Starmer's spokesman said that there was "an ongoing domestic Turkish legal process" and the UK "expects Turkey to uphold the rule of law".

If the mayor is not released, the CHP plans to keep fighting for the presidency.

"If they keep Ekrem Imamoglu locked up and hinder his candidacy," Ozel said, "any member of CHP can be a candidate and would be elected with 65% to 70%".