BARCELONABARCELONA (Reuters) - Around one million people filled central Barcelona on Tuesday to celebrate Catalonia's commemorative day and boost a bid for independence which has left deep divisions almost a year after it brought Spain to a constitutional crisis.

Tractors bearing red-and-yellow Catalan flags rolled into Barcelona from rural areas and separatists in red T-shirts bearing the slogan "Let's build the Republic!" chanted and sang.

The huge turnout, estimated by local police, was a response to a call from Catalan regional president Quim Torra and his predecessor Carles Puigdemont, who fled to Brussels last October after Madrid dismissed his government, to show continued support for independence from Spain.

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Socialist Prime Minister Pedro Sanchez, who took office in June, has taken a more conciliatory approach than his conservative predecessor Mariano Rajoy but has stood firm against a vote on or move towards independence.

Watching Tuesday's protest on the broad avenue that slices diagonally through Barcelona, 59-year-old psychologist Montse Martin said the movement needed to regain momentum.

"Let's see if from today there will be a turning point and we will be able to move forward," she said. "We have to get down to work and not focus so much on what has happened, as serious as it was, but look to the future."

The Sept. 11 "Diada" celebration marks the fall of Barcelona to Spain in 1714, and has been adopted by independence activists in recent years.

It falls just over a year after Puigdemont's administration held a referendum which Madrid sent riot police to try to stop, and made a unilateral declaration of independence.

Rajoy then imposed direct rule, on the basis of Spain's constitution, which states the country is indivisible.

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Torra and other attendees at the march wore yellow ribbons representing nine politicians and activists awaiting trial in jail for their role in the independence bid.

Their trials are expected to start later this year, a potential new source of tension. Sanchez told a Senate session on Tuesday that what Catalonia needed was "law and dialogue".

DEEP DIVISIONS

Divisions over the question of secession are stark in a region that makes up around one fifth of Spain's economic output and already has a high level of autonomy in areas including education and health, and its own police force.

A July poll showed 46.7 percent of Catalans saying they wanted an independent state while 44.9 percent did not. A separatist coalition won regional elections in December, although the fervently pro-unionist Ciudadanos emerged as the single largest party.

Ciudadanos's leader, Barcelona-born Albert Rivera, said on Twitter the Diada had been turned into "a day of exclusion, hatred and attack on Spain".

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Yellow-hued banners bearing the faces of those in jail and the word 'Free' hung from city lamp posts.

Maria Angels Brugada, waiting for friends at a central square ahead of an evening march, said the prisoners and the politicians who fled into exile would set the mood for this year's celebration.

"There are many people who aren't here and are without their families," said the 61-year-old. "It gives me goose pimples just talking about it."

(Writing by Isla Binnie and Sonya Dowsett; Editing by John Stonestreet)