The leaders of Spain's rival political parties joined voters at polling stations today in the nation's second general election in six months.

The anti-austerity party Podemos is expected to make big gains, potentially delivering a fresh jolt to Europe's political mainstream after Britain voted to leave the European Union.

The last election in December broke the mould of 40 years of stable conservative or Socialist majorities and failed to produce a viable government as upstart parties channelled growing resentment of the establishment following an economic crisis and a raft of corruption scandals.

Opinion polls suggest that the parliament that emerges this time will be just as fragmented as the previous one, with four big parties and six smaller regional ones winning seats in the 350-strong assembly, and none of them coming close to a majority.

The centre-right PP looks set to be the biggest party again, with around 120 seats, but its most natural potential coalition partner, the liberal Ciudadanos ("Citizens"), looks likely to win only about 40 seats, leaving them well short of the 176 needed for an absolute majority.

Yet the rise of Unidos Podemos ("Together We Can"), a far-left alliance led by Podemos, could in theory offer a way out.

The 90 seats it is expected to win, combined with around 80 for the Socialist Party (PSOE), would come close to an overall majority. Support from some of the regional parties could then allow them to form a government.

Many analysts believe, however, that the 137-year-old Socialist Party would prefer to form a 'grand coalition' with the People's Party (PP) of the current acting prime minister, Mariano Rajoy, or give passive support to a minority PP government, rather than get into bed with a group that threatens their existence.