The extent of the protests has varied day by day.
The ones that started on Saturday night were the biggest so far.
At least 6,000 people had gathered at the Plaza de Universitat in Barcelona in a demonstration that quickly degenerated.
Younger protesters in particular began throwing stones and other objects at police, who responded with batons and rubber bullets.
Storefronts were smashed along the city's luxury shopping street Passeig de Gràcia, and a large number of shops were looted.
Symbolic buildings such as the stock exchange and the Palau des Música have been vandalized, reports el País.
Barcelona Mayor Ada Colau has called for calm, while Spanish Interior Minister Miquel Sàmper regrets that what began on Tuesday as a protest against the "right to freedom of expression" has now degenerated and resulted in "acts of pure vandalism and looting of shops".
The clashes between perpetrators of violence and police forces have also intensified.
At least two people are being treated in hospital for their injuries.
A total of about thirty people have been arrested in the last 24 hours.
Most in Barcelona but also in Tarragona and Lleida, the Catalan city where the protests began since rapper Pablo Hásel barricaded himself at the university in an attempt to escape the prison sentence he was sentenced to after insulting Spain's royal house and praising terrorists.
Demonstrations were also taking place in cities elsewhere in Spain, including Madrid, reports AP.
There, however, the violence has brightened with its absence, except in Pamplona in Navarrra where protesters threw glass bottles at the police.