"We're going to work so that independence groups don't win," Rajoy told Spanish radio.
The Catalan independence crisis has triggered alarm in Brussels as the European Union deals with the fallout of Brexit and more than 2,400 businesses have moved their legal headquarters out of the region as uncertainty persists.
Rajoy dismissed the government of Catalan ex-leader Carles Puigdemont last month over his independence bid, suspending the regional parliament and organising a new election.
The prime minister has been rallying support for his Popular Party (PP) in the December 21 election in Catalonia -- a region that remains deeply divided over independence despite its parliament's declaration.
The PP only managed to finish fifth in Catalonia's 2015 election, which saw pro-separatist groups gain power in the region of 7.5 million people.
On Tuesday Rajoy issued "a call for massive participation" from voters on December 21 in the hope that parties in favour of keeping Catalonia part of Spain put in a strong showing.
Several former Catalan cabinet members are currently in jail over their role in agitating for independence, which is illegal under Spain's constitution.
Rajoy said that there was no ban on detained officials contesting the regional vote but added that they "need to respect the law".
"They can all run as candidates since they've not been declared ineligible" by a judge, Rajoy told COPE radio.
But he accused deposed Catalan officials of being "politically delegitimised" after "tricking Catalan citizens" by claiming independence.
Puigdemont himself is in self-imposed exile in Brussels and has said he wants to run as a candidate next month.
With fallout from the crisis affecting his own PDeCAT party's standing in polling, he had hoped to form a united separatist ticket with his former government ally, the leftwing ERC.
But the ERC said last week that it would not allow its candidates to run alongside PDeCAT hopefuls.
Puigdemont accuses Madrid of readying a "wave of repression" against separatists, but EU officials have staunchly backed Rajoy over the crisis.