The worker's legs were also injured when an oil barrel with chemical residue exploded on Wednesday morning and he had been rushed to the city's Souza Aguiar hospital, the union added.
"We must improve our working conditions which are not ideal. We'll only return to work when our demands have been heard," union director Romildo da Silva told Reuters.
Hundreds of workers held a demonstration outside the stadium to draw attention to their demands, which also include higher pay and health care. Up to 1,500 people work on the stadium in rotating shifts.
The contractor said the workers had "interrupted reforms at the Maracana in the early afternoon."
"These are labour issues that are being analysed by the consortium in charge of the work," Rio de Janeiro State Public Works said in a statement.
The famed Maracana will stage Confederations Cup matches in 2013 and is expected to host the World Cup final a year later. The renovations have been estimated at one billion reais ($630 million).
Brazil's World Cup preparations have previously been hit by industrial action.
In June, workers renovating the Mineirao stadium that will host World Cup matches in Belo Horizonte, capital of the southern central state of Minas Gerais, went on strike for four days demanding better pay and conditions.
