BARCELONA, Oct 29 (Reuters) - Hundreds of thousands of supporters of a unified Spain filled Barcelonas streets on Sunday in one of the biggest shows of force yet by the so-called silent majority that has watched as regional political leaders push for Catalan independence.

Political parties opposing a split by Catalonia from Spain had a small lead in an opinion poll published on Sunday, the first since Madrid called a regional election to try to resolve the countrys worst political crisis in four decades.

Polls and recent elections have shown that about half the electorate in the wealthy northeastern region, which is already autonomous, oppose secession from Spain, but a vocal independence movement has brought the current crisis to a head.

Spains central government called an election for Dec. 21 on Friday after sacking Catalonias president Carles Puigdemont, dissolving its parliament and dismissing its government. That followed the assemblys unilateral declaration of independence in a vote boycotted by three national parties.

The regional government claimed it had a mandate to push ahead with independence following an unofficial referendum on Oct. 1 which was ruled illegal under Spanish law and mostly boycotted by unionists.

Waving thousands of Spanish flags and singing Viva Espaa, protesters on Sunday turned out in the largest display of support for a united Spain since the beginning of the crisis underlining the depth of division in Catalonia itself.

Im here to defend Spanish unity and the law, said Alfonso Machado, 55, a salesman standing with a little girl with Spanish flags in her hair.

Knowing that in the end there wont be independence, I feel sorry for all the people tricked into thinking there could be and the divisions theyve driven through Catalan society.