Police in Honduras fired teargas Monday at demonstrators who set up flaming roadblocks and looted stores after the incumbent president was declared the winner of a heavily-disputed election.

The leftist opposition has claimed fraud in the election three weeks ago that finally led to the pronouncement Sunday of conservative President Juan Orlando Hernandez as the winner after an oft-interrupted vote count.

International observers also cited irregularities, and the defeated opposition candidate Salvador Nasralla arrived in Washington Monday to present what he said was evidence of electoral fraud to the Organization of American States (OAS).

"We are presenting multiple pieces of evidence, as the theft had various facets," said Nasralla, a 64-year-old former TV presenter.

Nasralla said that among those items were what he described as recordings of Hernandez ordering the head of the electoral commission, David Matamoros, not to release the election results until they had been doctored in his favor.

The opposition candidate was due to go on to meetings with the US State Department.

"The United States does not know the reality of the fraud and we are bringing a document (on a) USB showing how they suppressed the votes," he said.

Protests broke out Sunday night in Honduras and continued into Monday morning as police cleared the streets of barricades.

In the northern city of San Pedro Sula, police said protesters looted stores and burned a bank branch and a bus.

In the capital city Tegucigalpa, people woke up Monday to smoldering barricades of tires, sticks and rocks. Authorities sometimes needed heavy machinery to remove them. Under a light rain, some small groups of demonstrators continued to man roadblocks.

On a road leading out of the north of the city, police fired teargas at demonstrators and on a major thoroughfare they negotiated with protesters to leave one lane open to traffic.

- Fresh polls -

There has been violence since the November 26 election, as anti-Hernandez protesters and police squared off repeatedly.

Police have counted three deaths in the unrest. But the opposition says 20 people have died, and Amnesty International registered 14 fatalities.

The ballot was deeply contentious.

Hernandez, 49, stood for re-election against Nasralla, the candidate of the leftist Opposition Alliance Against the Dictatorship, despite a constitutional ban on presidents having more than one term.

His conservative National Party said that rule was scrapped by a 2015 Supreme Court ruling.

But the opposition insists ballots were tampered with after the election, and says unusual breaks in the count that dragged out the tally over more than a week were suspicious.

The leftist opposition alliance said it was not recognizing Hernandez's win.

Manuel Zelaya, a former president who was overthrown in a 2009 coup, said the country would not recognize Hernandez.

"We absolutely reject the declaration by the Supreme Electoral Commission ... We refuse to acknowledge any acts because the people recognize Salvador Nasralla as president," he said.

The Supreme Electoral Tribunal had previously declined to name a victor, despite saying that its count of the ballots showed a slight margin in favor of Hernandez: 43 percent to 41 percent for Nasralla.

But it had to do so by a December 26 deadline, or risk the entire election being invalidated.

Nasralla is insisting the entire vote be held again, with greater international scrutiny. The OAS secretary general, Luis Almagro, said his the organization's own observers had deemed the elections to be "poor quality" and backed calls for a fresh poll.

Honduras is beset by violence, poverty and corruption and provides many of the undocumented migrants headed to the United States.