Brazil's President and Workers' Party (PT) presidential candidate Dilma Rousseff (L) greets supporters during a campaign rally in Nova Iguacu near Rio de Janeiro October 20, 2014. [Photo/Agencies]

RIO DE JANEIRO/BRASILIA - President Dilma Rousseff is gaining momentum but remains locked in a dead heat with challenger Aecio Neves ahead of Sunday's runoff to Brazil's presidential election, two surveys showed on Monday.

Centrist candidate Neves lost a slight but statistically insignificant lead over leftist Rousseff for the first time since the first round of elections on Oct 5, according to the survey by Datafolha polling firm.

Since then, the presidential race has turned increasingly ugly as the candidates scramble for undecided voters. They have swapped accusations of corruption and economic mismanagement in heated televised debates over the past few days in what is shaping up to be Brazil's tightest election in decades.

The Datafolha poll showed the leftist incumbent with 46 percent of voter support, up 3 percentage points from the previous poll released on Oct 15. Support for market favorite Neves dropped 2 percentage points to 43 percent.

A survey by polling firm MDA released earlier on Monday showed Rousseff with 45.5 percent of voter support versus 44.5 percent for Neves.

The difference between the two remains within the margin of error of both polls.

Excluding undecided voters, spoiled and blank survey responses, the Datafolha poll showed Rousseff had 52 percent against 48 percent for Neves, a senator and former state governor.

Brazil's currency and stock markets extended losses earlier on Monday after the MDA poll showed Neves of the Brazilian Social Democracy Party lost momentum.

Brazilian markets have been extremely volatile throughout the campaign, surging with each poll favoring Neves and slumping with surveys that show Rousseff gaining ground. After four years of sluggish growth and heavy-handed economic policies under Rousseff, investors are eager for a new government.