Spanish police arrested 12 people Wednesday in raids on offices of the regional government of Catalonia, news reports said, intensifying a crackdown on the region's preparations for a secession vote that Spain says is illegal.

It was the first time Spanish authorities have detained Catalan officials since the campaign for a secession vote in Catalonia began to gather momentum in 2011.

Spain's Europa Press news agency and other media outlets said the raids mostly targeted Catalonia's economic and foreign departments as authorities worked to halt all preparatory moves for the referendum planned for Oct. 1.

Hundreds of people immediately began gathering to protest the raids and shout pro-independence slogans outside government offices in the region's capital, Barcelona.

The Catalan regional government confirmed Josep Maria Jove, secretary general of economic affairs, was among those arrested. Jove is number 2 to the region's vice president and economy chief, Oriol Junqueras.

Police and judicial authorities declined to give details on the operation because a judge has placed a secrecy order on it.

An Interior Ministry statement said only that Civil Guard police were carrying out an operation to gather evidence as part of investigations into the referendum's preparations.

Spanish Prime Minister Mariano Rajoy's conservative government is waging myriad legal battles to halt the referendum called by the pro-independence coalition ruling Catalonia.

Backed by most Spanish opposition parties, the government says the referendum violates the constitution and that if Catalonia wants a vote it must work to change the constitution first. The Constitutional Court has ordered the vote to be suspended as it studies its legality, but Catalan officials say they will press ahead regardless.

The Catalonia region centered on Barcelona generates a fifth of Spain's gross domestic product. It self-governs in several important areas, such as police, health and education. But key areas such as taxes, foreign affairs and most infrastructures are in the hands of the Spanish government. Both Catalan and Spanish are spoken in the region of 7.5 million people, and many Catalans feel strongly about their cultural heritage and traditions.

Hostility between Madrid and Barcelona has ramped up since Spain's Constitutional Court suspended the referendum, planned for Oct. 1, following a legal challenge by Prime Minister Mariano Rajoy. The Madrid government says the referendum contravenes the constitution, which states that Spain is indivisible.

Under Article 155 of Spain's constitution, Madrid has the power to intervene directly in the running of Catalonia's regional government, forcing it to drop the vote. This could involve sending in the police or suspending the regional government's authority to rule.

In February, the Constitutional Court ruled against the referendum and warned Catalan leaders they faced repercussions if they continued with their project.

In 2014, Catalonia held a non-binding vote under then President Artur Mas, in which more than 80 percent of those who cast a ballot chose independence, although just 2.3 million out of 6.3 million eligible voters took part. But in holding the symbolic referendum, Mas went against Spain's Constitutional Court, which had outlawed the vote, even if it was non-binding. He was later put on trial and banned from holding office for two years.

At the height of pro-independence fervor in 2012, during a deep economic recession in Spain, around one million people took to the streets waving the Catalan flag and singing the Catalan anthem. Polls have shown support for independence waning since then, and those wanting a separate state are in a minority. However, a majority of Catalans want to hold a referendum on the issue.