MADRID (Reuters) - The head of Spain's Socialist party, which has slipped to third place in polls ahead of parliamentary elections on Sunday, ruled out a coalition pact with either the center-right People's Party (PP) or the current leader of anti-austerity Podemos.

Some polls have indicated that a coalition between the Socialists and Podemos could win enough seats to take a majority and end a six-month political deadlock.

But Pedro Sanchez said in a radio interview on Monday: "We won't back or bet on any government led by the PP or (acting Prime Minister Mariano) Rajoy ... and (Podemos leader Pablo) Iglesias will not be prime minister."

Sunday's ballot follows an election in December that left no single party able to form a government and led to a succession of inter-party talks that failed to produce a viable coalition.

As it did six months ago, the PP is expected to win, but again falling well short of an overall majority.

Polls have shown that a joint ticket between Podemos and United Left (Unidos Podemos or 'Together We Can') would likely overtake the Socialists this time around as the main contender for Rajoy's PP.

Podemos finished third in the December ballot.

Sanchez has previously baulked at joining with Podemos, mostly due to the far-left group's support for an independence referendum in Catalonia.

He did not say whether he would be ready to back a coalition with Unidos Podemos not led by Iglesias, or if the Socialists would permit the PP to govern by abstaining in parliament.

Rajoy, in a television interview on Monday, reiterated that the Socialists should either stand aside and allow the PP to govern or join forces in a two-party "grand coalition".

(Reporting by Paul Day and Tomas Cobos; Editing by Julien Toyer and John Stonestreet)