At least 350,000 people gathered in Barcelona, capital of Catalonia, for a rally against independence from Spain.
They waved Spanish and Catalan flags and carried banners saying “Together we are stronger” and “Catalonia is Spain”.
It was the largest such rally in Catalonia amid the furore over last week’s disputed independence referendum.
Spanish PM Mariano Rajoy has warned he would not rule out anything “within the law” to halt Catalan secession.
Similar rallies were held across Spain on Saturday.
The final results from the referendum in the wealthy north-eastern region suggested 90% of the 2.3 million people who voted backed independence. Turnout was 43%.
There have been several claims of irregularities, and many ballot boxes were seized by Spanish police.
Nearly 900 people were injured as the police, trying to enforce a Spanish court ban on the vote, attempted to disperse voters.
Thirty-three police officers were also hurt.
Police in Barcelona said 350,000 people turned out for Sunday’s rally; organisers put the figure at 950,000.
Among them were former government minister Josep Borrell, who said his fellow Catalans needed to recover their level-headedness, and nobel laureate Mario Vargas Llosa.
“You need more than a coup plot to destroy what has been built over 500 years of history,” Vargas Llosa, the Peruvian-Spanish novelist, told the crowds.
One of those attending the rally was 72-year-old Araceli Ponze. She told Reuters: “We feel both Catalan and Spanish.
“We are facing a tremendous unknown. We will see what happens this week but we have to speak out very loudly so they know what we want.”
Similar unity rallies were held across Spain on Saturday.