MADRID (REUTERS) - Spain said on Thursday (Oct 19) it would suspend the autonomy of the wealthy northeastern region of Catalonia over its bid to secede, pitching the euro zone's fourth-largest economy into uncharted political waters.

Barcelona, the capital of a region which produces a fifth of Spain's economic output, is now heading for direct rule from Madrid for the first time in Spain's 40 years of democracy.

Here is what could happen in the next few days:

ARTICLE 155

Prime Minister Mariano Rajoy will hold a Cabinet meeting on Saturday at which he will trigger Article 155 of the 1978 constitution, which allows the government to take control of any of the country's 17 autonomous regions if they break the law.

No national leader has ever activated this measure before, but Rajoy says its use is justified now because Catalonia has fallen foul of the law by pressing ahead with a bid for secession following a banned referendum on Oct 1.

The legislation is vague but theoretically allows Madrid to sack the local administration and install a new team, take control of police forces and finances, and call a snap election.

Its use could spur more wrangling with the rebel region, but Rajoy says he has broad backing from other parties, and has spurned all Catalan president Carles Puigdemont's calls for negotiations.

The main opposition Socialist party says it fully supports the government but will insist Article 155 is applied"proportionately".

The government says it would not suspend the autonomy in an outright manner but instead make specific requirements of the regional government to respect the law.

This would in any case take a few days to become effective once the article is invoked. A supporting vote is needed from the upper house Senate, where Rajoy's People's Party (PP) has a majority of seats.

REGIONAL ELECTION

Most observers believe the crisis will only be resolved with regional elections.

Rajoy could replace the regional government with a new rank of politicians or technocrats, with a view to holding fresh elections within three months, Spanish media have reported.

Alternatively Puigdemont could call elections himself in a bid to avoid a situation where individual regional departments could be taken over by delegates from central ministries. While regional president he would remain nominally in his role but stripped of any power, Catalan newspaper La Vanguardia said.

CATALAN COALITION CONUNDRUM

Puigdemont has ignored Rajoy's demands to clarify an ambiguous declaration of independence he made on Oct 10, and says he might put secession to a formal vote in the Catalan parliament if Madrid holds firm.

At home, he faces pressure to declare independence from within his own coalition, including from the second-largest party Esquerra Republicana.

The far-left, pro-independence CUP has only 10 members of parliament but could bring down the government if they retract support, automatically leading to regional elections.

HARD LINE

After a police crackdown on the Oct 1 vote was widely condemned, Rajoy needs to tread carefully, but he appears to have support among many Spaniards for taking a hard line.

According to a closely watched monthly official poll, concern among Spaniards over the situation in Catalonia tripled in September, which began with the regional parliament voting to hold a referendum on the matter, from the month before.

And while a majority say they have both national and regional sentiments, 66 per cent believe Rajoy is right in not offering more autonomy to the country's 17 autonomous regions, the same poll showed.

When it comes to voters of his People's Party, this number jumps to 86 per cent.

But fears of social unrest on the other side of the political spectrum remain. Activists have gathered peacefully in Barcelona to protest the jailing of the leaders of two pro-independence organisations accused of sedition, and new protests have been called for Saturday.