* Catalonia will hold election on Dec. 21
* Puigdemont says all parties should unite against Madrid
* Electoral alliance must be registered by end of Tuesday (Adds a Puigdemont quote and background)
MADRID, Nov 7 (Reuters) - Catalonia's deposed leader Carles Puigdemont urged the region's political forces on Tuesday to unite against Spain, as a window for him to seal an electoral pact with other pro-independence parties began to close.
Puigdemont went into self-imposed exile in Belgium last month after Spain's central government fired his secessionist administration, dissolved the Catalan parliament and called an election in the region for Dec 21.
Pro-secession parties want that vote to become a de facto independence referendum. Two of those parties, Puigdemont's PDeCAT and the ERC party, said at the weekend they might contest it on a combined ticket.
But they must register any alliance by the end of Tuesday, and prospects of them bridging their differences in time looked slim.
The Catalan independence push has deeply divided Spain, dragging it into its worst political crisis since the return of democracy four decades ago and fueling anti-Spanish sentiment in Catalonia and nationalist tendencies elsewhere.
In an interview with Catalunya Radio on Tuesday from Brussels, Puigdemont said all parties contesting the election should unite against Madrid.
"The ideal would be a broad regional list of parties... that stand for democracy and freedom." he said, mentioning PDeCAT, ERC, the anti-capitalist CUP and left-wing +Podemos.
ERC's spokesman Sergi Sabria said on Monday his party did not rule out a coalition with PDeCAT, but would agree only if other parties joined them, including CUP, which has yet to decide whether it will contest the December ballot.
Unequivocal support for Puigdemont and his cause came from some 200 pro-independence Catalan mayors who attended a rally in Brussels on Tuesday evening. They called for continued non-violent resistance and for the European Union to intervene.
"Perhaps the path towards freedom will be longer than we thought, but it continues and we haven't given up," said Toni Comin, one of four former cabinet members in Brussels.
Polls show the ERC and PDeCAT combined would not win enough votes for a majority in the Catalan parliament, though running together would increase their chance of success.
Puigdemont also said he might be in jail by the time of the election, "but prison doesn't deprive anyone of legitimacy."
Madrid issued an arrest warrant against Puigdemont on charges including rebellion, but a Brussels court ruled on Monday the deposed leader could remain at liberty in Belgium until it had decided whether he should be extradited.
He and other secessionist leaders face the charges for organizing an independence referendum on Oct. 1 and proclaiming a Catalan republic, in defiance of Spain's constitution.
The party that forms the main opposition to the secessionists in Catalonia emerged as the big winner in the first nationwide voter survey published by Spain's most closely watched polling group since the referendum.
Support for the pro-business Ciudadanos rose almost three percentage points to 17.5 percent, the Sociological Research Centre (CIS) survey showed.
Podemos -- which supports a negotiated referendum on independence in Catalonia -- and its allies fell almost two points to 18.5 percent in the survey.
During Tuesday's rally, Puigdemont called on the Spanish government to suspend Article 155, which Prime Minister Mariano Rajoy enacted last month to govern Catalonia from Madrid, and said they would contest it in the December election.
"Catalans have the right to know if their choice will be respected on Dec. 21," he said. (Additional reporting by Jesus Aguado and Angus Berwick in Madrid; Writing by John Stonestreet; Editing by Gareth Jones)