Catalan separatists claimed a mandate to start breaking away from Spain after they won a majority of the seats in the regional parliament on Sunday. But without a majority of the votes, they will likely be challenged by opposition parties in Catalonia and the central government in Madrid.

Regional president Artur Mas, whose separatist Together for Yes alliance was projected to win 62 out of 135 seats in the legislature, promised supporters in Barcelona he would press ahead with a plan to declare independence from Spain in eighteen months. He vowed that the effort would be carried out “with a sense of integration inside Catalonia and a sense of harmony with regard to Spain and Europe.”

But Spain’s central government has ruled out independence, saying the country’s laws do not allow for any region to break away. The European Commission has also called into doubt the separatists’ claim that Catalonia would remain part of the European Union after seceding.

Mas had cast the regional elections on Sunday as a de facto referendum on independence after Spain’s highest court last year declared a planned plebiscite illegitimate.

Repeated attempts to frustrate Catalan nationalism have fueled support for independence. As recently as 2010, only one in five Catalans wanted to break away from Spain. The turning point came when the Constitutional Court threw out most of the region’s autonomy statute that year. It also said the definition of Catalonia as a “nation” had no legal standing.

The election’s high stakes drove a record four million Catalans to the polls.

Bu the high turnout — 77 percent — did not primarily benefit Mas’ party.

In alliance with the Republican Left, his Convergència Democràtica de Catalunya actually lost nine seats.

The far-left Popular Unity Candidacy (CUP) took the rest of the pro-independence vote. It went up from three to ten seats and could give Mas a majority, even though it has only governed at the municipal level so far.

Combined, the Together for Yes alliance and the CUP got 48 percent support.

On the anti-independence side, the centrist Ciudadanos (Citizens) replaced the conservative People’s Party and Socialists as leaders of the opposition. The two mainstream parties — which are still the largest in Spain as a whole — went down from 39 to 27 seats. The Ciudadanos picked up half a million more votes and went up from nine to 25 seats.

On the far left, an alliance led by the anti-establishment party Podemos, which also opposes Catalan independence, disappointed. It won the Barcelona municipal elections in May but only takes eleven seats in the regional parliament, two fewer than the far left won in 2012.

The standoff between what is Spain’s richest region and its central government is likely to continue at least until December when the country as a whole votes in parliamentary elections. With neither the ruling People’s Party nor the Socialists projected to win a majority, the Ciudadanos could become kingmakers in Madrid — and influence how the state deals with its restive northeastern province.