It was the second death at the Arena Amazonia this year, and the third fatality in a World Cup stadium in less than a month.
A few hours after Saturday's death, another worker died of a heart attack while working outside the venue in Manaus, and a local union threatened to start a strike there on Monday to complain about inadequate conditions offered to labourers.
In late November, two workers were killed when a crane collapsed as it was hoisting a 500-ton piece of roofing at the Sao Paulo stadium that will host the tournament's June 12 opener. Last year, a worker died at the construction site of the stadium in the nation's capital, Brasilia.
Another worker died in April at the new Palmeiras stadium, which may be used for teams training for games in Sao Paulo.
Brazil had already made headlines a week ago because of fan violence in the final round of the Brazilian league, and again earlier this month after World Cup organizers announced that none of the six stadiums that had to be finalized by the end of the year would be delivered on time.
Andrade Gutierrez, the construction company building the Arena Amazonia, said in a statement Saturday that 22-year-old Marcleudo de Melo Ferreira fell some 115 feet (35 metres) in the early morning accident — the second fatality at the venue since construction began in 2010. Another man died there in March.
The 49-year-old man who had a heart attack was paving an area outside the venue when he died. He was working for a construction company hired by local officials, and local media reported that family members complained that he was working seven days a week and there was pressure from his superiors to finish the work on time.
After the worker's fall earlier in the day, a local union said it was considering a strike to call attention to inappropriate working conditions.
"We need a strike to show what is really happening inside the arena," Cicero Custodio, president of Amazonas state civil construction workers' union, told GloboEsporte.com.
The Manaus stadium will host four World Cup matches, beginning with England vs. Italy on June 14. It will also host the United States vs. Portugal on June 22.
Most teams were hoping to avoid playing in Manaus because of humid and hot conditions in the jungle city, as well as the increased travel distance. After complaints from England coach Roy Hodgson before the World Cup draw earlier this month, Manaus Mayor Arthur Virgilio said he hoped "to get a better team and a coach who is more sensible and polite."
When The Associated Press visited the stadium this week, workers were installing diamond shaped panels to the latticework of steel girders that form part of the stadium roof. Dozens of labourers were balanced on the girders as they worked. When complete, the panels on the roof are meant to resemble snake scales.
Andrade Gutierrez said the causes of the accident would be investigated but reiterated its commitment to worker safety.
The local World Cup organizing committee said work on the Manaus stadium was halted for a period of mourning and will resume on Sunday.
"FIFA and the Local Organizing Committee (LOC) learned of the death of the worker on Saturday at the Arena Amazonia site with great sadness," World Cup organizers said in a statement. "We would like to send our most sincere condolences to his family, relatives, colleagues and friends."
Manaus officials said they aim to hold the first match to test the stadium on Jan. 15, with the 10,000 or so workers who participated in the stadium's construction serving as spectators. Delays in the unblocking of federal financing for the venue slowed construction, causing FIFA to push back its original December deadline for stadium delivery.
After the deaths in Sao Paulo in late November, FIFA said "the safety of workers is the top priority" for football's governing body and local organizers.
In early October, a Brazilian labour judge halted work at the Arena da Baixada in Curitiba for nearly a week because of safety concerns. Workers there went on strike this week over late pay. The stadium in the southern Brazilian city is one of the most delayed among the six that still need to be delivered to FIFA.
Associated Press writer Stan Lehman contributed to this report from Sao Paulo and AP Sports Writer John Leicester contributed from Curitiba, Brazil.