The Spanish government is reinforcing security at airports and rail stations in Catalonia before a meeting which could see the region declare independence.
Catalan President Carles Puigdemont is due to address the regional parliament on Tuesday afternoon (local time).
The announcement will come following a contentious referendum on the region's future, which was marred by police violence.
European Council President Donald Tusk Puigdemont not to declare independence.
Tusk, a former Polish prime minister who chairs EU summits and seeks to reflect of the European Union's governments, make the comments during a speech to an EU regional forum in Brussels on Tuesday.
"I ask you to respect, in your intentions, the constitutional order and not to announce a decision that would make such a dialogue impossible. 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."
Tusk, who noted his own ethnic minority background and personal experience of being hit by police batons as an anti-Communist activist in the 1980s, said he had also urged Spanish Prime Minister Mariano Rajoy to "look for a solution to the problem without the use of force".
The intervention, just hours before a speech in Barcelona in which Puigdemont could make a unilateral declaration of independence, was a dramatic shift for the EU, where the executive Commission has called for dialogue in the crisis but has stopped short of a clear rejection of Catalan independence, describing it as an "internal matter" for Spain.