The U.S.-based non-profit group, a pioneer of international election observation since the 1980s, said that both the winning leftist candidate Luiz Inacio Lula da Silva and far-right President Jair Bolsonaro were targets of disinformation.

However, "our analysis of the leading fact-checking organizations indicates that most attacks targeted the Lula campaign," it said in a statement wrapping up its mission on observing the Brazilian election.

The Carter Center sent a mission of experts to the South American country at the invitation of the electoral authority, the Superior Electoral Court, or TSE, which invited a record number of foreign observers as a way to tackle repeated criticism by Bolsonaro of the electronic voting system.

Bolsonaro has argued without evidence that the system is vulnerable to fraud and, in the weeks prior to the runoff vote, stepped up his attacks on electoral officials.

Until the first round of the vote, on Oct. 2, the dominant disinformation narrative centered around flaws of the voting system, the group said, adding that the court provided an extensive set of audit procedures to address the claims.

"Responding to suggestions of auditors in previous electoral processes, the courts have successively increased the scope of these audits and reduced restrictions for auditors," it said.

In the second round of the election, the focus shifted to questioning the impartiality of the TSE, the Carter Center said.

The TSE also investigated misinformation production and distribution, as well as their financing mechanisms.

In the final weeks of the campaign, the court decided to expand its ability to rapidly remove content from social media platforms, a move the Carter Center said "raised concerns about interference with fundamental rights."

Lula, a former president, narrowly won the run-off by 50.9% against 49.1% for Bolsonaro in Brazil's most divisive election in decades.