The race is tightening between Bolivia's two main presidential candidates, according to an opinion poll released on Friday, though many voters said they were still undecided with little more than a week ahead of the crucial vote.

Socialist candidate Luis Arce leads centrist former President Carlos Mesa by just six percentage points, the poll showed, indicating a dip in support for the MAS political party of Bolivia's socialist former president Evo Morales.

The October 18 election will be key to the direction of the landlocked country after a fraught ballot last year prompted Morales' resignation and plunged the Andean country into turmoil.

The opinion poll of 15,500 Bolivians was carried out between October 2 and 5 by "Your Vote Counts," a group of several universities and local media. It showed that 33.6% of voters said they would support Arce, down from 40.3% in a September poll, and no longer enough to win the election outright. Just under 10% of people polled, however, said they were still undecided.

Mesa's support came in at 26.8%, up slightly from 26.2% in September. Conservative interim President Jeanine Anez, who had about 10.6% voter support, withdrew her candidacy last month in a bid to unify the opposition against Arce, a former economy minister and Morales loyalist.

To avoid a second round, the election winner requires at least 40% of valid votes in the first round and a ten point advantage over the closest competitor.

Political analyst Alvaro del Rio said the poll indicates that the election will likely advance to a second round.

"It clarifies a lot," Del Rio said. "The latest polls show us that the second round is practically Bolivia's decision. No party will be able to gain an advantage of ten points and reach 40% to avoid a second round."