King Felipe criticised Catalonian authorities for showing "disrespect" on a televised speech on Tuesday.
Spain was braced for further political upheaval Wednesday after Catalonia's leader said the region would declare independence "in a matter of days". He's also addressing the regional parliament Monday to review the disputed vote last Sunday - a session that his parliamentary supporters in the radical CUP group say will consider the independence declaration.
Felipe said the referendum's organisers showed "disrespect to the powers of the state", but made no mention of Sunday's violence. "So it is fundamental that the constitutions of every one of our member states are upheld and respected".
Even when calling for dialogue, European leaders have sided with Spain.
But he said that Spain "will overcome hard times".
Wary of interfering in Spain's domestic affairs, the EU representatives called for talks between the government in Madrid and Catalan authorities, but shied away from suggesting that the bloc could play a peacemaking role, despite appeals from Catalonia for European mediation. Spain's interior minister said the 5,000 extra officers would stay in Catalonia as long as necessary.
Opinion polls conducted before the vote suggested only a minority of around 40 percent of residents in the region back independence although a majority want a referendum to be held.
Catalonians find a contradiction in European values when it comes to its support for the independence of Scotland and Gibraltar from United Kingdom while it avoids the demands of their region.
CUP is not a part of the Catalan government, which is formed by two mainstream separatist parties.
The Catalan government said the vote in support of independence was almost 90%, but official results have not yet been released.
Barcelona's port was at a standstill, trade union sources said.
"If somebody tries to declare the independence of part of the territory - something that can not be done - we will have to do everything possible to apply the law", Catalá said on national television Monday.
Spain's central government does not recognise the vote as legitimate and it has been condemned as illegal by the European Commission.
The Catalan government, in turn, insists that it will only negotiate with Madrid if a legally binding referendum is on the table, something that the Spanish government opposes.