BARCELONA, Spain — More than half a million pro-independence Catalans on Friday took over one of Barcelona’s main avenues, as their separatist leaders began their latest attempt to break loose from Spain.

The march was held on Catalonia’s national day, but it was also the start of campaigning for a regional parliamentary election on Sept. 27 that Catalonia’s main separatist parties are presenting as a de facto vote on independence after agreeing to run as a joint list of candidates.

Speaking here on Friday ahead of the march, Artur Mas, the Catalan leader, said the election was a make-or-break chance for Catalonia to become a new European state. He called on the Madrid government to grant Catalonia the right to secede if separatist parties secure a parliamentary majority.

“We cannot lose this opportunity,” Mr. Mas said during a joint news conference with the other main separatist leaders. “When you take a risk, you can win or you can lose, but if we accept the defeat, we also ask that others accept the victory.”