Any release on bail would mark a turn in the campaign for regional elections on December 21st, particularly for separatist parties who have repeatedly accused Madrid of taking “political prisoners” and “repression” after their attempt to declare unilateral independence failed.
“State repression is the mobilising element of the independence movement right now,” said Oriol Bartomeus, politics professor at the Autonomous University of Barcelona.
“Once they no longer have political prisoners, they will have to change their campaign.”
Those who will appear before Judge Pablo Llarena of the Supreme Court Friday are Catalonia's deposed vice-president Oriol Junqueras, seven other former regional ministers and the leaders of two pro-independence civic associations.
They were due to appear at 9.30 am (0830 GMT) in the Madrid court, which has taken on the investigation into Catalan leaders, most of whom are accused of rebellion, sedition and misuse of public funds.
Until last week, Spain's National Court, which takes on major criminal cases, had been in charge of the case and had sent Junqueras and his former ministers to jail pending the probe on November 2nd.
Before that, the two leaders of the pro-independence ANC and Omnium Cultural associations, Jordi Sanchez and Jordi Cuixart, had also been sent to prison by the same court.
However, Llarena of the Supreme Court decided the members of Catalonia's parliament he had been investigating on similar charges could remain free as the probe continues.
Now he has taken on the probe of the other separatist leaders, they hope he will make the same decision for them.
The jailing of separatist leaders has caused outrage in Catalonia, where pro-independence supporters have organised rallies and wear yellow ribbons as a sign of solidarity.
Only Thursday, Catalonia's deposed president Carles Puigdemont, who fled to Belgium after the independence declaration, said it was “disgraceful and indecent” that they were in jail.
Puigdemont and several other former regional ministers have remained in Belgium, where they await possible extradition to Spain.
Despite facing charges that could carry up to 30 years in jail, Puigdemont, Junqueras and the majority of the Catalan government that was sacked by Prime Minister Mariano Rajoy after the independence declaration will take part in regional elections.
But after the failure of declaring independence, they have not clarified how they will proceed if they win elections again like they did in September 2015 as part of a coalition.
The release of the separatist leaders could also revive tensions within the separatist bloc, particularly between Puigdemont and Junqueras — from a conservative and left-wing party respectively.
Unlike the 2015 elections, Puigdemont's PDeCAT party and Junqueras's ERC will not join forces as part of a coalition.
They are standing separately and competing to lead the independence movement.
“They are forcing independence supporters to choose between a jailed martyr vice-president or a president in exile,” said Gabriel Colome, a politics professor at the Autonomous University of Barcelona.
ERC is currently favourite to win the elections later this month according to opinion polls, which also predict separatist and anti-independence parties will be neck-and-neck.