Spanish Prime Minister Mariano Rajoy has urged Catalans to oust separatists from their regional parliament in the early election he has called for next month.

Mr Rajoy has told members of his conservative Popular Party in Barcelona that "we want a massive turnout to open up a new period of normalcy".

His visit to Catalonia's main city was his first to the region since he used extraordinary powers to stifle its secession push.

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After Catalonia's parliament voted on October 27 in favour of a declaration of independence, Mr Rajoy responded by firing its government, dissolving the parliament and calling the early election for December 21.

He said: "It's urgent to return a sense of normality to Catalonia and do so as soon as possible to lower the social and economic tensions."

The Prime Minister's calls came as hundreds of thousands of people backing Catalonia's bid to secede from Spain packed the streets in of central Barcelona to demand the release of jailed separatist leaders.

The rally's grassroots organisers called for 10 prominent members of the secessionist movement in the north-eastern Spanish region to be freed from prison.

Eight former members of Catalonia's dissolved Cabinet and two activists are in jail while Spanish authorities investigate their alleged roles in promoting an illegal declaration of independence last month in violation of Spain's Constitution.

A separate court in Madrid granted bail on Thursday to another six Catalan MPs who are the subject of another investigation into the secession push.

Barcelona police said that 750,000 people attended the rally.

Many of the protesters carried pro-independence "estelada" flags, with its white star and blue triangle superimposed over the traditional red-and-yellow Catalan colours.

Many also held signs saying in Catalan "Freedom Political Prisoners" and wore yellow ribbons as a symbol of their demands.

Family members of the jailed separatists read messages from their loved ones to the crowd at the conclusion of the march.

Grassroots group National Catalan Assembly organised more than 500 buses to bring people from towns and villages across Catalonia to its main city of Barcelona.

Also on Saturday, the pro-independence Republic Left party announced that its jailed leader Oriol Junqueras will be its top candidate for the upcoming regional elections on December 21.

The Catalan party is including other jailed leaders in its list for the regional parliament. Polls show that Republic Left is favoured to win the upcoming ballot, although it will not secure an outright majority.

The Catalan conflict is the worst constitutional crisis to threaten Spain in nearly four decades.

A day after Catalonia's parliament voted in favour of a declaration of independence on October 27, Spain's government activated extraordinary powers given to it by the Senate to dismiss the region's government, dissolve its parliament and call local elections.

While those separatist leaders now in jail obeyed a summons to appear in court in Madrid, deposed Catalan president Carles Puigdemont and four of his former ministers fled to Belgium, where they now await an extradition hearing to return them to Spain.

Mr Puigdemont and his fellow separatists claim that a referendum on secession held on October 1 gave them a mandate for independence, even though it had been prohibited by the nation's highest court, failed to meet international standards and was boycotted by anti-independence parties.

Less than half of the electorate turned out to vote, and the referendum was also disrupted by brutal police raids.

No foreign power has recognised Catalonia's claim to independence.

The European Union has warned that an independent Catalonia would be cast out of the 28-nation bloc.

The most recent regional elections and opinion polls show that Catalonia's 7.5 million residents are roughly split over remaining a part of Spain or going their own way.

Most pro-independence supporters feel that the Catalan language and culture would have a better chance of flourishing in a separate state and that their economic prospects would be improved.

The business sector has so far not been convinced, with more than 2,000 companies transferring their headquarters out of the north-eastern region in recent weeks for fears of being pushed out of the common EU market.

The Spanish Constitution says the nation is "indivisible" and that questions of national sovereignty should be addressed by the national parliament in Madrid.