EU President Donald Tusk on Tuesday urged Catalonia's separatist leader not to take a decision that would make dialogue with Madrid "impossible" and respect the constitution, just hours before a possible declaration of independence.

Brussels has firmly backed Madrid in the escalating standoff over the banned referendum vote for Catalan independence on October 1 but called for talks to defuse the crisis, which has sent shockwaves through the European Union.

Catalan government chief Carles Puigdemont is due to address an extraordinary session of the region's parliament which starts at 1600 GMT, though it remains unclear whether he will follow through on a threat to announce a full breakaway from Spain.

"Today I ask you to respect in your intentions the constitutional order and not to announce a decision that would make such a dialogue impossible," Tusk said in Brussels.

Tusk, a self-confessed former football hooligan, said he made his appeal in a spirit of European unity and also "as a man who knows what it feels like to be hit by a police baton".

"Diversity should not, and need not, lead to conflict, whose consequences would obviously be bad: for the Catalans, for Spain and for the whole of Europe. Let us always look for what unites us, and not for what divides us," Tusk said.

His comments echoed a warning from Madrid to Puigdemont "not to do anything irreversible" and not to make a unilateral declaration of independence.

Puigdemont says the referendum result -- 90 percent in favour of independence -- justifies splitting from Madrid, though the poll was poorly monitored and many Catalans opposed to secession simply stayed home. Turnout was just over 42 percent.

The EU has insisted the crisis is an internal matter for Spain and defended Madrid's right to use "proportionate" force after dramatic images of riot police clashing with voters on polling day caused international alarm.

At stake is the future of a region of 7.5 million people deeply divided over independence, one of Spain's economic powerhouses whose drive to break away has raised concern for stability in the European Union.

Source: AFP