* Organisers say protest represents "silent majority"
* Catalan leader could declare independence on Tuesday
* Spain PM Rajoy doesn't rule out suspending Catalan autonomy
* Abertis is latest firm to consider moving head office (Adds estimate of crowd size, quotes)
BARCELONA, Oct 8 (Reuters) - Hundreds of thousands of people took to the streets of Catalonia's capital Barcelona on Sunday to express their opposition to declaring independence from Spain, showing how divided the region is on the issue.
A crowd estimated by local police to number 350,000 waved Spanish and Catalan flags and carried banners saying "Catalonia is Spain" and "Together we are stronger". They poured into the city centre after politicians on both sides hardened their positions in the country's worst political crisis for decades.
Spanish Prime Minister Mariano Rajoy said on Saturday he would not rule out removing Catalonia's government and calling a fresh local election if it claimed independence, as well as suspending the wealthy region's existing autonomous status.
The stark warning came days before Catalan leader Carles Puigdemont is expected to address the region's parliament, on Tuesday, when he could unilaterally declare independence.
The anti-independence demonstration, which included Catalans and people from other parts of Spain, underlined how the dispute has riven the region itself, coming less than a month after a million people rallied in the same city to support independence.
"We feel both Catalan and Spanish," Araceli Ponze, 72, said during Sunday's rally. "We are facing a tremendous unknown. We will see what happens this week but we have to speak out very loudly so they know what we want."
The northeastern region of 7.5 million people, which has its own language and culture, held an independence referendum on Oct. 1 in defiance of a Spanish court ban.
More than 90 percent of the 2.3 million people who voted backed secession, according to Catalan officials. But that turnout represented only 43 percent of the region's 5.3 million eligible voters as many opponents of independence stayed away.
The Spanish government sent thousands of national police to the region to prevent the vote. About 900 people were injured when officers fired rubber bullets and charged crowds with truncheons in scenes that shocked Spain and the world, and dramatically escalated the dispute.
Losing Catalonia is almost unthinkable for the Spanish government.
It would deprive Spain of about 16 percent of its people, a fifth of its economic output and more than a quarter of its exports. Catalonia is also the top destination for foreign tourists, attracting about a quarter of Spain's total.
There is widespread opposition to a Catalan breakaway among people in the rest of the country.
The political stand-off has pushed banks and companies to move their headquarters outside Catalonia. The board of Catalonia-based infrastructure firm Abertis will meet on Monday to discuss moving its head office elsewhere in Spain, a source familiar with the matter said.
Concern is growing in EU capitals about the impact of the crisis on the Spanish economy, the fourth largest in the euro zone, and on possible spillovers to other economies.
Some European officials are also worried that any softening in Spain's stance towards Catalan independence could fuel secessionist feelings among other groups in Europe such as Belgium's Flemings and Italy's Lombards.
Until this weekend, Rajoy has remained vague on whether he would take the unprecedented step of triggering Article 155 of the constitution, the so-called nuclear option which enables him to sack the regional government and call a local election.
However, asked if he was ready to do so, Rajoy told El Pais newspaper on Saturday: "I don't rule out anything that is within the law ... Ideally, we shouldn't have to take drastic solutions but for that not to happen there would have to be changes."
Rajoy also said he planned to leave in Catalonia the 4,000 national police the government had shipped in for the referendum, until the crisis was over.
He ruled out using mediators to resolve the crisis - something Puigdemont has said he is open to - and added the issue would not force a snap national election.
Sunday's demonstration in Barcelona was organised by the anti-independence group Catalan Civil Society to mobilise what it believes is a "silent majority" of citizens in Catalonia who oppose independence.
"The people who have come to demonstrate don't feel Catalan so much as Spanish," said 40-year-old engineer Raul Briones, wearing a Spanish national soccer team shirt. "We like how things have been up until now and want to go on like this."
The rally was addressed by Nobel prize-winning novelist Mario Vargas Llosa, who has dual Spanish and Peruvian nationality. He told reporters it showed many Catalans "don't want the coup d'etat the Catalan government is fostering".
Tens of thousands of people had gathered in 50 cities across Spain on Saturday, some defending Spain's national unity and others dressed in white and calling for talks to defuse the crisis. (Additional reporting by Raquel Castillo and Robert Hetz in Madrid, Elisabeth O'Leary in Edinburgh and Madeline Chambers in Berlin; Writing by Adrian Croft; Editing by Pravin Char)