Their decision followed a banned referendum on the subject, which separatists say provided a mandate for independence but which Spanish courts ruled to be illegal.
Spanish Prime Minister Mariano Rajoy and fellow opponents of Catalan independence, meanwhile, have hitched their hopes on a record turnout on December 21 to return a legislature in favour of unity with Spain.
Puigdemont and the other separatist leaders claim a mandate for independence from the October 1 referendum, which was boycotted by parties against secession.
ERC was part of the former Catalan ruling coalition with Puigdemont's conservative party, but now leads the polls ahead of the new election.
The former leader of the Catalonia government, Carles Puigdemont, and four of his disbanded cabinet, are in self-imposed exile in Belgium under conditional release after an worldwide arrest warrant was served against them.
Yesterday, Paul Bekaert, Puigdemont's defence lawyer, argued that the Spanish charges, which carry punishments of decades in prison, are not punishable in Belgium, and so grounds for extradition do not apply.
Seven of the imprisoned politicians are candidates with separatist parties.
"We also highlighted the danger for the impediment of their human rights in Spain", he said.
Catalans have plenty of local parties to choose from, along with the local branches of the big national parties, and opinion polls suggest that fragmentation will not make securing an absolute majority easy for the separatists.
Rovira said the jailing "is a very clear attempt to win these elections without political adversaries".
Junqueras, who was removed as Catalonia's vice president by the Spanish government along with other top officials, is leading the ERC party's ticket in the December 21 election.
The CIS said the poll had a margin of error of 1.8 percentage points.
Before campaigning officially started, Catalan pro-independence groups held protests in front of town halls to oppose the Supreme Court's decision to keep some of the independence movement's leaders in custody.
Eight former Catalan officials jailed near Madrid have pledged to give up unilateral independence for the wealthy region in the hope of being freed. All Catalan politicians have faced charges of rebellion and sedition.
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.