President Jair Bolsonaro had yet to concede defeat in Brazil’s presidential election as of Monday afternoon, prompting fears he may challenge the victory of his leftist rival, former president Luiz Inacio Lula da Silva, Reuters reported.

Tens of thousands of jubilant Lula supporters took to the streets of Sao Paulo on Sunday night to celebrate the stunning return of the 77-year-old former metallurgist, who ruled Brazil from 2003 to 2010.

His election victory came after a stint in prison on corruption charges that were later overturned.

Bolsonaro, who left his residence on Monday morning and headed to the presidential palace, has yet to make any public comments. But before voting, he repeatedly made baseless statements that the electoral system was open to falsifications.

Several pro-Bolsonaro truckers blocked Brazil’s highways to protest Lula’s victory, and some called for a military coup.

Truck drivers loyal to Bolsonaro are known for wreaking economic havoc in Brazil, a major food exporter, when they shut down highways.

The roadblocks, which authorities said had covered 12 Brazilian states by morning, have not yet disrupted grain exports, but could do so if they continue.

Lula has promised to reverse many of Bolsonaro’s policies, including measures on guns and lax protections for the Amazon forest.

Environmentalists, world leaders and sustainability investors hailed Lula’s victory and his commitment to protecting rainforests and restoring Brazil’s leadership in the fight against climate change.

Shortly before taking office on January 1, President-elect Lula will send representatives to the United Nations COP27 climate summit in Saudi Arabia next month, Marina Silva, a union environmentalist, said on Monday.

In his victory speech on Sunday night, Lula pledged to fight the illegal logging, mining and land grabbing that has led to the increasing destruction of the Amazon forest over the past four years under Bolsonaro.