BARCELONA—On Thursday, tech entrepreneur Rafael Pous will take his place among an expected one million Catalan residents lining up to form a seven-mile long "V," for vote.

The display is intended to symbolize this wealthy industrial region's determination to hold a nonbinding referendum on possible independence from Spain, despite staunch opposition to such a vote from the government in Madrid.

The late-afternoon event—held on what Catalans celebrate as their "National Day"—reflects the determination of grass-roots activists like Mr. Pous. They have transformed the idea of Catalan independence from a fringe position to a mass movement that poses a threat to Spain's stability, much as Scotland's campaign for independence has unsettled the U.K.

Mr. Pous has trooped to 50 town meetings up and down Catalonia during the past couple of years, showing his PowerPoint presentation about why the region should split from Spain, even quoting from the U.S. Declaration of Independence.

His message is simple and, in a country with an unemployment rate of nearly 25%, almost irresistible: "It doesn't have to be this bad," he says. "We can do things much better on our own."