SEOUL (Reuters) - A pro-North Korea candidate from a far-left party quit South Korea's presidential race on Sunday and threw her weight behind the main opposition candidate in a move that could decide the outcome of an election that appears too close to call.

Lee Jung-hee, who was polling up to 1.6 percent in some surveys, stood aside ahead of the final presidential debate between conservative Park Geun-hye and Moon Jae-in.

Park leads Moon by 0.5-2.0 percentage points in the most recent polls. The election is on Wednesday.

Lee has acted as a spoiler in the debates so far, making strident attacks on Park, the daughter of South Korean dictator Park Chung-hee.

Lee told a news conference that she had decided to quit "to meet the expectations of the whole nation" in a bid to change the presidency, currently held by conservative Lee Myung-bak whose single mandatory five-year term is coming to an end.

She attacked Park again on Sunday saying that the 60-year-old was part of an "old and corrupt dictatorship" and that an election victory for her would be a "disaster" for South Korea.

South Korea votes under the shadow of North Korea's recent successful rocket launch, although economic and welfare issues are the main concerns in the election.

In a poll by South Korean broadcaster SBS, the North was the top concern for just 4.7 percent of voters.

The North on Sunday held a memorial service for former leader Kim Jong-il ahead of the December 17 first anniversary of his death.

The former leader was praised for his "military first" policies and for turning North Korea into "a world-level military power and legitimate nuclear weapons state", state news agency KCNA reported.

There are concerns that the impoverished and isolated North, now led by the late leader's son, 29-year old Kim Jong-un, could follow its rocket launch with a nuclear test, as it did in 2009.

(Reporting by Sung-won Shim; Editing by David Chance and Robert Birsel)