Catalans calling themselves a "silent majority" opposed to leaving Spain broke their silence after a week of mounting anxiety over Spain's worst political crisis in a generation.
Hundreds of people gathered on the city's Urquinaona square waving red and yellow flags and singing "Viva Espana".
"We have perhaps been silent too long," Alejandro Marcos, 44, told AFP.
"It seems that the one who yells the most wins the argument. So we have to raise our voices and say loud and clear that we do not want independence."
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"The unity of Spain cannot be voted on or negotiated -- it must be defended," read one sign in the crowd.
Recent polls had indicated that Catalans are split on independence, though leaders said the violence during the referendum turned many against the state authorities.
On the eve of the rally, Spain's Prime Minister Mariano Rajoy issued a stern warning to Catalan leaders who have said they could declare independence this week.
"I rule out absolutely nothing that is allowed for under the law," he said in an interview published in El Pais newspaper.
"The ideal would be not to have to take drastic measures," he said.
"I would like this threat of a declaration of independence to be withdrawn as quickly as possible."
Yesterday's rally came exactly one week after the contested vote that has triggered Spain's worst political crisis in a generation.
The Barcelona protest -- organised by the Societat Civil Catalana, the main anti-independence group in Catalonia -- was scheduled to get formally underway at noon (1000 GMT).
Yesterday tens of thousands of demonstrators, many dressed in white, hit the streets of Madrid and other cities across Spain to demand dialogue to end the dispute.
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