Catalonia's parliament voted on Friday to declare independence from Spain and proclaim a republic, just as Madrid is poised to impose direct rule on the semi-autonomous region to stop it in its tracks.

A motion declaring independence was approved with 70 votes in favour, 10 against and two abstentions, with Catalan opposition MPs walking out of the 135-seat chamber before the vote in protest at a declaration unlikely to be given official recognition by Madrid and abroad.

Tens of thousands of independence supporters massed near the Catalan parliament cheered with joy after lawmakers' vote.

Watching proceedings in parliament on two large screens, they clapped and shouted “independence” in Catalan before singing the regional hymn, many raising their fists.

Catalan President Carles Puigdemont has called on fellow separatists to remain peaceful ahead of an expected crackdown by Spanish authorities.

Facing a crowd of hundreds of supporters packing Catalonia's parliament building, he said: “In the days ahead we must keep to our values of pacificism and dignity. It's in our, in your hands to build the republic."

"Today the Parliament fulfilled the long-desired and fought-for step and culminated the mandate of the ballot boxes,” he added, referring to a banned referendum separatists held on October 1.

Those gathered then erupted into the Catalan anthem “Els Segadors” (The Reapers) and chants of “Liberty!”

Spanish Prime Minister Mariano Rajoy to “restore legality” in Catalonia after the regional parliament declared independence.

“I ask all Spaniards to remain calm. The rule of law will restore legality in Catalonia,” he wrote in a Twitter message immediately after the Catalan assembly voted in favour of declaring independence from Spain.

A spokesman with Spain's prosecutor office says that the country's top prosecutor will seek rebellion charges for those responsible for a vote in favour of declaring an independent Catalan republic.

The spokesman said the prosecutor is looking to determine if the charges should be limited to the Catalan cabinet, including Puigdemont and Vice President Oriol Junqueras, or if they should also include members of the parliament's governing board and lawmakers.

The official, who spoke under condition of anonymity in line with internal rules, said the charges could be brought as early as Monday.

Under Spanish criminal law, rebellion can be punished with up to 25 years in prison, with shorter term penalties if the act of rebellion doesn't lead to violence.

The wealthy region's drive to break away from Spain has unleashed the country's worst political crisis in a generation.

After staging a banned referendum on October 1, in which separatists said 90 per cent of people voted for secession, Puigdemont had said he had accepted a “mandate for Catalonia to become an independent state”.