Madrid (CNN) -- Hundreds of thousands of Catalans in northeastern Spain are due to increase pressure on Madrid for an independent, breakaway state Wednesday by forming a human chain for 400 kilometers (about 250 miles).
The human chain is organized by the grass-roots, citizen-led Catalan National Assembly, which last year on September 11 -- Catalonia's national day -- turned out an estimated 1.5 million people in Barcelona, the regional capital.
Catalan politicians followed up by demanding a referendum on self-determination by the end of 2014, which the Spanish government in Madrid staunchly opposes.
While the political battle continues between Spain's two largest cities, Madrid and Barcelona, the human chain has emerged as the latest rallying point.
Organizers say 370,000 Catalans have signed up to take part, and they predict tens of thousands more will also participate. The chain will traverse Catalonia from the north, near the French border, to the south, on its border with the Spanish region of Valencia.
"It will be an innocent but powerful image to push the process ahead, holding hands," said Alfred Bosch, who represents Catalonia's pro-independence Republican Left party in Spanish Parliament in Madrid.
Dubbed the "Catalan Way Toward Independence," the human chain shows the process can only "go ahead, forward, and not back," Bosch said.
But the Spanish government doubts that. It says that Catalonia, with 7.5 million people, already has broad home-rule powers, including its own parliament, police force and control over education and health.
And Madrid insists that the Spanish Constitution does not allow any of Spain's 17 regions to unilaterally break away, even one like Catalonia that has its own flag and language.
On the eve of the human chain, Spanish Foreign Minister Jose Manuel Garcia-Margallo said political leaders should find a way to keep Catalonia within Spain.
But Catalan's president, Artur Mas, in an opinion article published Wednesday in The New York Times, wrote: "We ... seek no harm to Spain. We are bound together by geography, history and our people, as more than 40 percent of Catalonia's population came from other parts of Spain or has close family ties. We want to be Spain's brother, as equal partners."
Catalonia says it's been the junior partner for too long. It produces 19% of Spain's wealth and says it sends far more in taxes to Madrid than it gets back in central government spending. It recalls a long history of slights, and at certain times outright repression, by Spain.
The human chain will start at 17:14 local time (11:14 a.m. ET), to honor Catalans who on September 11, 1714, lost a decisive battle to Spanish troops. The chain will last about an hour.
The current polemic is not just between Catalonia and Spain. Within Catalonia, there is also tension between leading political forces over the timing, and the potential wording, of the referendum.
President Mas, whose center-right Convergence and Union party governs only because of support from the Republican Left, seemed to leave the door open last week for a vote on self-determination later than 2014, perhaps in 2016. He has since repeated that he favors the 2014 deadline.
But the Republican Left and those organizing the human chain insist it must be by the end of 2014, with or without the consent of Madrid.
Various opinion polls show a very large majority of Catalans want the right of self-determination. But if independence makes it to the ballot, polls show the result could be tighter, some predicting a victory in the 50% range.