Jamaica's ruling party appeared headed for defeat in national elections on Thursday, as the Caribbean country's leading opposition party capitalized on voter discontent over growing economic woes.

Preliminary results showed the governing Jamaica Labour Party, or JLP, winning just 21 of the 63 parliamentary seats at stake in the national election.

With about 50 percent of ballots counted, the opposition People's National Party, or PNP, looked set to win 42 seats, according to results from the national electoral office posted on the website of the Jamaica Observer newspaper.

The moderately left-leaning PNP is led by Portia Simpson Miller, a former prime minister who became Jamaica's first female leader in 2006 and now looks set to ride a wave of popular disillusionment back into office.

There was no immediate of claim of victory from Simpson Miller, but PNP campaign spokesman Franklyn Delano said the people had spoken at the polls and preliminary results left no room for doubt about their message.

We have been returned (to power), Delano told national television as PNP supporters cheered wildly at a rally in the background.

The people on the ground are clearly saying to us that what we have experienced in the last 4 1/2 years we would not wish to continue, he said.

On the eve of Thursday's balloting, polls showed Jamaica's two dominant parties running neck and neck in a vote focused on the island's stagnant and debt-ridden economy.

But if the preliminary trend holds up, and is confirmed later by the Electoral Commission of Jamaica, Prime Minister Andrew Holness would become one of the shortest-serving premiers in the island nation's history.

Holness, 39, took office in October after the center-right JLP suffered a blow when his predecessor surprisingly resigned. He had hoped to keep the center-right JLP in power for a second consecutive term.

SADDLED WITH DEBT

Although one of the Caribbean's more developed economies, Jamaica is saddled with a public debt load totaling more than 120 percent of gross domestic product.

Its burdensome debt has proved a drag on the economy, which is dependent on tourism and has failed to grow over the past four years, sputtering since the JLP took power.

Unemployment has risen to 12.9 percent from 9.8 percent in 2007.

Analysts say the new government will likely be forced to implement unpopular austerity measures, including possible layoffs of state workers, in an effort to shore up the economy after it received a $1.27 billion lifeline from the International Monetary Fund last year.

Holness pledged on the campaign trail to spur the economy by attracting private investment to infrastructure projects. He also said the ruling party had reduced crime.

Simpson Miller vowed if elected to appeal to the IMF to extend the period Jamaica has to repay any loans to hand authorities more leeway to jump-start the economy.

(Writing by Tom Brown; Editing by Peter Cooney)