Spain's Guardia Civil police have detained a senior Catalan official and raided regional government ministries involved in organising a banned independence vote. BBC reported
Tensions were already high before the arrest of Josep Maria Jové, secretary-general of the Catalan vice presidency.
Catalan leaders are defying a court order to halt the vote, condemned by the Madrid government as illegal.
One official called for peaceful resistance to protect the buildings.
"The time has come - let's resist peacefully; let's come out and defend our institutions," the president of the Catalan National Assembly, Jordi Sánchez, tweeted.
'We will not allow it'
The economy, foreign affairs and presidency buildings were all targeted early on Wednesday, 11 days before the referendum.
The detained official's boss, Catalan Vice-President Oriol Junqueras, accused Spanish police of attacking the region's institutions and therefore its citizens too. "We will not allow it," he said.
The night before, Spanish police discovered a mass of documents directly related to the banned vote.
Catalan police officers, on patrol outside the building in Terrassa, scuffled with pro-secession protesters trying to block the street outside.
The Catalan government is trying to organise the 1 October referendum, in the face of determined resistance by the national government to prevent it going ahead.
The Madrid government has been backed up by Spain's Constitutional Court, which suspended the referendum law passed by the Catalan parliament.
Some 7.5 million people live in Spain's well-off north-eastern region. Although opinion polls have been rare, one survey commissioned by the Catalan government in July suggested that 41% of voters backed independence while 49% were opposed.
One of the most important aims for the national authorities is to stop voting cards being sent out in the first place.