Analysts say a turnout of less than 75 percent would harm the ruling Socialists, and that a turnout below 70 percent could tip the scales to a victory for the conservative opposition Popular Party. Neither big party is likely to be able to govern without some form of support from regional or leftist parties, with whom the Socialists have made ad hoc alliances over the last four years. Sunday's election is overshadowed by the killing of a former Socialist councillor in the Basque Country which authorities blamed on Basque separatist group ETA.

Voting is under way in Spain's general election in a ballot expected to return the ruling Socialists to power.

But smaller parties may hold the balance in a close race between Jose Luis Rodriguez Zapatero's governing party and the conservative opposition Popular Party.

Election campaigning was dominated by worries over soaring immigration and the end of an economic boom.

Campaigning was cut short on Friday after Isaias Carrasco, a former city councillor was shot dead in the Basque town of Mondragon.

Authorities have blamed Basque separatist group ETA for the killing with many saying it was an attempt by the group to influence Sunday's poll.

Security was high in Mondragon as hundreds gathered outside the Town Hall for Carrasco's funeral.

Four years ago, then opposition leader Zapatero won on a wave of voter anger at the ruling Popular Party (PP), who tried to blame the Madrid train bombings on ETA.

Political commentators say this time around, the attack on Carrasco is unlikely to radically change the poll's outcome.

Some say it may boost turn out and that could be crucial for the Socialists whose supporters are less reliable voters.

Michelle Carlile-Alkhouri, Reuters.