Catalans have massed in a vast human chain stretching hundreds of kilometres along the Mediterranean coast, demanding independence from Spain in the face of fierce opposition from the national government.
Hundreds of thousands of people in yellow T-shirts joined in, according to organisers, yelling "Independence!"
They linked hands along highways and through towns in the northeastern region, amid countless pro-independence flags bearing the red and yellow stripes of Catalonia overlaid with a white star on a blue background.
Proud of their Catalan language and culture, but suffering now in a recession, many of the 7.5 million people in debt-laden Catalonia say they feel slighted and short-changed by the central government which redistributes their taxes.
"We need to put an end to the cultural and economic suffocation we are suffering," said Carme Forcadell, president of the Catalan National Assembly, the grassroots group organising the human chain.
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"We have come out in our hundreds of thousands into the street to show in a democratic and inclusive way that we are capable of achieving any aim we set ourselves," she told the crowd in central Barcelona.
She said earlier that the rally aimed to draw 400,000 people and to stretch for 400km along Catalonia's Mediterranean coast.
The chain passed by landmarks such as the Sagrada Familia basilica in Barcelona and the city's Camp Nou football stadium.
They joined hands at 5.14pm as rain showers gave way to sunshine.
The timing was a reference to the year 1714, the date of the military defeat which for many Catalan nationalists marked the beginning of oppression by the Spanish state.
Wednesday's protest marked Catalonia's national day, the Diada, which recalls the conquest of Barcelona by Spanish king Philip V's forces that year.
