Puigdemont's rallying cry came a day after a Spanish judge issued an arrest warrant against him, with prosecutors seeking to charge him with rebellion, sedition and misuse of public funds over his secessionist drive.
"The time has come for all democrats to unite. For Catalonia, for the release of political prisoners and for the republic," he said on Twitter.
The 54-year-old has been holed up in Belgium since Monday and ignored a summons to appear before the judge in Madrid, saying he wants guarantees he will receive a fair trial.
The Belgian public prosecutor's office confirmed Saturday that it had received the warrants, saying a decision would happen within 24 hours of them appearing before a judge.
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But the authorities said any appeals process could last for up to three months.
"The EU Framework Decision provides that the final decision must be taken within 60 days, with an extension to 90 days under exceptional circumstances," the Belgian justice ministry said in a statement.
Protesters in the wealthy northeastern region have held frequent demonstrations, chanting and waving Catalan flags while calling for the officials' release, and a big pro- independence march is planned for November 11.
Spain's deepest political crisis in decades flared up after the regional parliament in Catalonia voted to proclaim an independent republic following a referendum on October 1 that was declared illegal by the country's Constitutional Court.
In a tweet, he referred to an online petition calling for the creation of a combined independent candidate list, which by today afternoon had received over 50,000 signatures.
During the last regional election in September 2015, Puigdemont's conservative PdeCat and the far-left ERC party stood together in a "Together for Yes" coalition.
But there have been growing tensions between the two allies over strategy and it is not clear if they will stand together again.
"It's absolutely indispensable that we have a joint strategy to battle the repression," Sergi Sabria, a spokesman for the separatist ERC party, told Catalunya Radio, in a sign the two main parties might work together.
Told of Puigdemont's unity call, the ERC's deputy leader, Marta Rovira, responded simply: "We must win (the elections) and that is what we will do."
The leaders of another separatist party, the far-left Popular Unity Candidacy (CUP), met today in the southern French city of Perpignan and said they would make an announcement regarding the elections on November 12.
Parties that wish to stand together in a coalition in the polls must announce this by November 7.
Whether Puigdemont and his colleagues in Brussels, Maria Serret Aleu, Antoni Comin Oliveres, Luis Puig Gordi and Clara Ponsati Obiols, will be able to take part in the election is an open question.
The Belgian justice ministry said there were "some situations" where European arrest warrants can be refused, but added: "If the decision is to execute the (warrant), the person is in principle surrendered to the authorities of the issuing state within 10 days following the decision."
Even though Catalonia is one of Spain's wealthiest regions, Spain's central bank has warned of a possible recession in the region, where unemployment rose sharply in October.
Despite the negative economic headwinds, Peter Ceretti at the Economist Intelligence Unit said pro-independence parties might win the December election, as the jailed ministers could deliver an "important propaganda" boost.