King Felipe of Spain has been met with protests on his first visit to Catalonia since last October's failed independence bid in the Spanish region.
Police held back hundreds of people as the king arrived in Barcelona for the Mobile World Congress technology fair.
Senior Catalan officials refused to attend a formal reception.
Meanwhile, Manchester City manager Pep Guardiola vowed to carry on wearing a yellow ribbon in support of imprisoned Catalan politicians.
Guardiola, a former Barcelona midfielder, said this was because he was a "human being before a manager".
In a setback for the pro-independence cause, a survey published by the respected Catalan Centre for Opinion Studies last week suggested support for independence had dropped sharply, with a majority of Catalans now against the idea.
According to the poll, 53.9% reject independence while 40.8% support it - down from 48.7% in October.
On Saturday, protesters in Barcelona chanted anti-monarchist slogans as King Felipe arrived for the opening of the fair.
Scuffles broke out between the demonstrators and riot police.
Senior Catalan officials, including Barcelona Mayoress Ada Colau, refused to attend a formal reception.
She said she was protesting against the king's support for the repression of the independence movement.
Pro-independence parties in the wealthy north-eastern region called a referendum on the issue in October, which was met with a heavy police crackdown and attracted global attention.
The government in Madrid sacked the Catalan regional government, imposed direct rule and called new elections, but pro-independence parties returned with a slim majority.
It is Spain's biggest political crisis since democracy was restored in 1975.
Several Catalan pro-independence politicians have been imprisoned in connection with the referendum, deemed illegal by Madrid.
Others, including sacked regional President Carles Puigdemont, are in self-imposed exile in Brussels.