A Spanish judge has ordered that four prominent members of the Catalan independence movement should remain in custody, including the vice-president of the ousted regional government.
He will now campaign in a polarised regional election from a jail near Madrid.
The judge set €100,000 bail for the six other Catalan politicians who had been jailed in early November, and ordered their passports to be confiscated.
The six were expected to leave jails near Madrid later in the day.
Meanwhile, Catalan ex-president Carles Puigdemont and four of his separatist allies heard that they will be judged on whether they can be extradited from Belgium to Spain on December 14, exactly one week before the election.
The group is refusing to return to Spain to face rebellion, sedition and embezzlement charges that can be punished with decades in prison under the country's criminal laws.
Mr Puigdemont's defence lawyer, Paul Bekaert, insisted that the Spanish charges were not punishable in Belgium and thus there were no grounds for extradition.
"We also highlighted the danger for the impediment of their human rights in Spain," he said.
Whatever decision is made on December 14, two appeals will be possible and a final ruling could well come only after the December 21 election called by Spain's central authorities, in which Mr Puigdemont is leading his pro-independence party's campaign.
Mariano Rajoy's conservative Spanish government disbanded Mr Puigdemont's Cabinet when it took control of Catalonia shortly after separatist regional politicians passed a declaration of independence in late October.
The early election is an attempt to find a democratic way out of the nation's worst crisis in nearly four decades.
But the vote is shaping up as a plebiscite between those for and against independence, with polls predicting a close race between the two camps.
Adding to the uncertainty, a Supreme Court judge decided on Monday to uphold the preventive jailing of ousted Catalan Vice President Oriol Junqueras, who tops the list for the left-republican ERC party.
ERC was part of the former Catalan ruling coalition with Mr Puigdemont's conservative party and is leading the polls before the new election.
Campaigning officially begins at midnight on Monday, and in the hours before that moment Catalan pro-independence groups hope to stage protests in front of town halls across the region against the Supreme Court's decision on the detained separatists.
Mr Junqueras and the other jailed politicians pledged last week to give up on efforts to seek unilateral independence for the wealthy northeastern region, in the hope of being freed.
But judge Pablo Llarena said in his decision on Monday that it remains to be seen if Mr Junqueras' pledge to abide by Spanish law is "truthful and real".
ERC spokeswoman Marta Rovira described the jailing as "a covert attempt" by Spain's central authorities in Madrid to get ERC out of the picture.
"This is a very clear attempt to win these elections without political adversaries," she said.
Spanish Interior Minister Juan Ignacio Zoido was unmoved by the arguments of Mr Junqueras' supporters.
"Those who commit criminal acts must place themselves at the mercy of legal rulings," Mr Zoido said.
The magistrate also upheld custody orders without bail for the former regional interior minister, Joaquim Forn, and the leaders of Assemblea Nacional Catalan and Omnium Cultural, the two Catalan grassroots groups that have been the main drivers of the separatist bid in Catalonia.
A government-run poll published on Monday indicated that pro-independence parties would lose their slim majority in the Catalan regional parliament.
It gave ERC, Mr Puigdemont's Junts per Catalunya (Together for Catalonia) and the far-left anti-establishment CUP party 66 or 67 seats out of the parliamentary 135 seats.
The election could be a close race between the left republican ERC party and the opposition business-friendly Ciutadans (Citizens), with 21 and 22.5% share of votes respectively, the CIS survey suggested.