Spain's Supreme Court convicted 12 former Catalan separatist leaders on Monday for their roles in a 2017 secession bid, triggering calls for mass protests in the affluent northeastern region.

In a landmark ruling, nine politicians have been ordered to serve between nine and 13 years in prison for sedition following the region's failed attempt at independence.

The court sentenced Oriol Junqueras, the former regional vice president of Catalonia, to 13 years for sedition and misuse of public funds.

The remaining three were found guilty only of disobedience and not sentenced to prison. All 12 were acquitted of the most serious charge of rebellion.

The former Catalan president Carles Puigdemont, who is on a self-imposed exile in Belgium, criticized the verdict in a tweet.

"One hundred years in all. An outrage. Now more than ever, by your side and those of your families. It is time to react as never before," he wrote on Twitter, adding: "For the future of our sons and daughters. For democracy. For Europe. For Catalonia."

In a letter, Junqueras, who was the main defendant in Puigdemont's absence, told supporters: "To those who are only driven by the will to do harm, we say to them that nothing ends today, you neither win nor convince. We will come back stronger… and win."

Spanish Prime Minister Pedro Sanchez released his own video defending the decision, saying Spain is one of the world's "most free and safe" countries.

News of the sentencing brought hundreds of protesters out on the streets in different parts of Catalonia and its capital, Barcelona.

Separatist groups had warned of "peaceful civil disobedience" if any of the politicians were found guilty.

Pro-independence activists have urged for a rally and plans are afoot for a march from five towns towards Barcelona, where a congregation would be held on Friday. A general strike has also been called on that day.

Spanish authorities have acted swiftly to pre-empt protests by deploying extra police officers to the region.

The Supreme Court judgment marks another important chapter in Catalonia's long struggle for independence from Spain.

Madrid is adamant that cannot be allowed.

The Catalan government held a referendum on 1 October 2017, ignoring the country's constitutional court, which called it illegal.

Despite the "Yes" vote winning, the count was deemed of dubious value due to the illegal nature of the exercise and low voter turnout.

The Catalan parliament, however, went on to declare independence three weeks later, embroiling Spain in one of its worst political crises in decades.